Question: Who are the Cylons?

Cylon (reimagining)




Cylons are a fictional artificially-intelligent "species" envisioned in the Battlestar Galactica science fiction series and related franchises. Originally created to serve human needs like other machines, a series of events involving the transfer of a human's consciousness into a Cylon's neural network led Cylons to evolve into sentient, self-aware beings. The original Cylons were purely mechanical.
The Cylon was reimagined in the second Battlestar Galactica television series and its prequel Caprica. The first configurations (Centurions) were robots. Others were cyborgs whose different configurations incorporated biological elements, including some that were nearly indistinguishable from humans.

Background
One of the series’ core themes is ‘All of this has happened before and will happen again’.[1] Cylon Centurions were invented twice: Once thousands of years before the events of Battlestar Galactica and later they fought a war against the Thirteenth Tribe on Earth.[2]
The second Centurion creation is detailed in the prequel series Caprica. There they were created by Zoe Graystone and her father Daniel Graystone, owner of Graystone Industries.[3]
Caprica
Centurions
The Caprica Cylon is a combination of programming done by Zoe and Daniel's MCP-enhanced ("Meta-Cognitive Processor") Graystone Industries U-87 Robot. A common activity on Caprica was to participate in a virtual world (V-World) that allowed players to live an alternate existence. The original Cylon was developed as a place to house Zoe's avatar (virtual existence) after she is killed in a bombing on a maglev train in Caprica Episode 1. Her father sees her avatar in V-World and forces her best friend Lacy to take him to her there where she explains that she is much more than a simple digital simulation. She has Zoe's memories from a neural scan Zoe developed to extend a V-World character into a virtual instance of the person. The digital Zoe says to her father, "I don't feel like a copy", and she becomes the basis of the first Cylons.
Cylons were originally accepted into Caprican culture as robotic workers after a group of them (under direct remote control by Graystone himself, with the help of his wife Amanda) successfully thwarted an attack on the public at Atlas Stadium by the Soldiers of the One (STO). The Cylons then spread throughout the colonies as a worker class, but because they had basic self-awareness they became increasingly resentful of their servitude.
Battlestar Galactica
In Battlestar Galactica, the Cylons were created thousands of years before the series began. They destroyed the Thirteenth Tribe living on the original Earth 2,000 years before.
BSGr Cylon society consists of three classes: biological ruling-class androids (pejoratively "skinjobs"), labor-warrior class (Centurions), and a transport-air warrior class (Basestars and Raiders). Basestars and Raiders are partly biological cyborgs and as such are vulnerable to biological threats such as disease.[4] Raiders and Centurions, while intelligent, were inhibited from exercising free will, although the Hybrids that controlled Basestars did exercise some autonomy and were respected by the skinjobs.[5]
The skinjobs lead Cylon society. Some are sleeper agents, programmed with false memories to think they are human. Cylons can convincingly interact with humans and are capable of intimate relationships.
They have limited superhuman abilities such as increased stamina and strength and the ability to connect with computer systems. Cylons are more sensitive than humans to some forms of radiation and more resistant than humans to others. They can upload their memories to other copies of their model through a central station. Another Cylon, of the same model, can then access these memories. The humanoid Cylons have the ability to distinguish between copies by sight, unlike Centurions and humans.
Toasters
Centurions are often called toasters by humans, partly a reference to their metallic construction but also to demean them. "Toaster" was also used to derogate skinjob, another derogatory term (and an homage to the film Blade Runner in which BSGr star Edward James Olmos has a supporting role). Centurions were also termed "chromejobs" or "bulletheads". The (chronological) origin of the word "toaster" can be found in the prequel Caprica, episode 10 which was created after BSG had ended. After a car crash made the first-ever test Cylon inoperable, it was considered "toast" after which Tomas Vergis joked "or a toaster". The term "skinjob" is originally used in Caprica, episode 17, where a new body is being discussed for an avatar (and AI) of "Zoe". The parents emphasize it will be "with skin" (as opposed to the Cylon, metal prototypes) which she then calls "a skinjob".
Reproduction
Cylons were initially unable to produce children with each other. They turned their attention to mating with humans. Their crossbreeding experiments in facilities called "farms" are unsuccessful. They theorize that love is necessary for them to procreate, because "God is love" in their theology. This leads to a successful experiment in which an Eight and a human, Karl Agathon, fall in love and produce a daughter named Hera.
Skinjobs can download their consciousness into another body when the body they are using dies, giving them virtual immortality. These downloads have a limited range, requiring the Cylons to build a "Resurrection Ship" to capture them. If not within range of a Resurrection Ship, downloading is unavailable and a Cylon's death is permanent. The downloading process is shared by the semi-organic Raider spacecraft, which allows Raiders who have been killed several times to become more formidable fighters ("Scar"). The Centurions do not download. Resurrection Ships function as boosters for the central Resurrection Hub. Without the Hub resurrection is not possible.
This technology was first shown in Caprica in an attempt to "resurrect" deceased relatives in a so-called "resurrection" program (first called that way in episode 13) in virtual reality where one could meet and interact with those one was mourning for.
Projection
Cylons have an ability referred to as "projection", a form of realistic daydreaming that allows them to change the appearance of their surroundings in their own mind (e.g. making the corridors of a ship look like a forest, etc.). They have the ability to touch, smell, etc. the fantasy environment. They can share projections and create virtual individuals as part of the fantasy. This ability allows them to express their subconscious desires.[6]
Boxing
If a Cylon is deemed dangerous by the other models, he or she may be "boxed" – deactivated with consciousness and memories placed in storage. This may involve a particular copy or an entire line. Boxed consciousnesses are stored in the Resurrection Hub and can be unboxed.  The term was also used in the prequel Caprica episode 10 which discusses "boxing" a defunct test cylon (as literally placing it in a crate for "deep storage"). As Caprica is the prequel this is chronologically the first time of its use from which the Cylons derived their own definition.
History

Thirteenth Tribe
Between 3–4,000 years before the Second Cylon War, it is implied that the humans and Cylons of Kobol fought each other, but the nature of the conflict remains unrevealed. The Thirteenth Tribe were Cylons who fled Kobol and settled on a planet they dubbed Earth. Eventually, they developed the ability to reproduce sexually. Resurrection technology fell into disuse and was lost. The Final Five redeveloped resurrection, following warnings by mysterious "angels" that a Kobol-like disaster was coming. When life on Cylon Earth was destroyed in a nuclear war between the thirteenth tribe and their mechanical Cylon creations (stemming from the latter's maltreatment at the hands of the Tribe), the Final Five downloaded into an orbiting vessel.[2]
The Final Five wanted to warn the other Twelve Colonies to treat their Cylon workers well to prevent another catastrophe. However, without faster than light (FTL) travel, they required approximately 2,000 years to make the journey (relative to human time, while only a few years passed inside the ship due to Special Relativity). When they arrived, the Twelve Colonies' Cylon Centurions had revolted and the first Cylon War was raging. It lasted roughly twelve and a half years.[2]
The Final Five learned that these Centurions were trying to develop organic bodies through experiments on humans, resulting in the first Hybrid. To end the war, the Final Five helped the Centurions develop skinjob model numbers One to Eight (model number Seven's creation was sabotaged by One) and resurrection technology. The war ended with an armistice and an attempt by the Colonies to establish diplomatic relations between the parties. Instead, the Cylons disappeared for forty years.[2]
The first of the Colonial skinjobs, One John Cavil, refused to let the Final Five's concern for humanity blind them. He boxed the Final Five. He then periodically sent them to the Colonies to see how terrible humanity was.[2]
Second Cylon War
After forty years, the Colonial Admiralty sent Battlestar Valkyrie on a covert mission close to the armistice line to gather intelligence on Cylon activity. A stealth ship from the Valkyrie crossed the armistice line, but was intercepted by the Cylons. Three years later the Cylons initiated a surprise attack on the Colonies. The attack was successful because a Cylon agent, later known as Caprica Six, infiltrated Caprica's colonial defense network with the unwitting complicity of renowned scientist Gaius Baltar and created backdoor programs to shut down the network and its defenses. The attacks wiped out billions of humans, nearly the entire colonial population. Two Battlestars – Galactica and Pegasus – survived. A fleet of civilian ships was scattered throughout the neighboring space. Together they fled into deep space.
The Cylons pursued them while initiating the next phase of their evolution, procreation. Female human survivors were detained and used in experiments to create Cylon-human hybrids. The experiments were unsuccessful leading the Cylons to conclude that the missing component was love. They tested this by using an Eight posing as Lt. Sharon "Boomer" Valerii from Galactica to seduce a marooned Galactica officer, Lt. Karl "Helo" Agathon on Caprica. They fell in love. The Eight abandoned the Cylons, helping Helo to escape. The couple later produced the first viable human/Cylon birth. This Eight joined the fleet with Helo. She later married Helo, joined the Colonial military and received the call sign "Athena".
Ten months after the initial attacks, Eight Sharon Valerii (call sign "Boomer"), attempted to assassinate Commander Adama under the influence of unexplained programing unknown to her. She was unaware that she was a Cylon before the attack, though she had been uneasy because of unexplained blackouts (during various attempts to sabotage Galactica). A vengeful crew mate, Cally Henderson killed her. She downloaded into a new body and settled on Cylon-occupied Caprica in her former apartment, unable to relinquish her human identity. She led a campaign for better treatment of the humans. She and other like-minded Cylons influenced the Cylon civilization, which withdrew from the colonial home worlds and pursued benevolent treatment of the humans and then reconciliation.
During this time, the half-human half-Cylon hybrid, Hera Agathon was born on Galactica. Fearing that the Cylons might capture the child, President Laura Roslin faked Hera's death and secretly had her adopted by a human woman.
New Caprica
The humans settled on a barren planet they dubbed New Caprica. The refuge lasted a year before the Cylons found them. The Cylons occupied the new human settlement, at first peacefully but later forcefully and more viciously as the human Resistance increased. In the end they used punitive methods to keep the humans in line, including summary executions and infiltration by seemingly sympathetic Cylons. One such infiltrator was an Eight model who manipulated Felix Gaeta into giving her a list of people the Resistance desired to free from Cylon detention whom she then had executed.
Escape
Four months later the colonials escaped with the help of a resistance movement and the efforts of both the Galactica and Pegasus. The escape required Athena's help. She entered the Cylon facility and took the keys to the various Colonial landing craft. Prior to this a Three found out from a human oracle that Hera was alive and on the planet. She rescued Hera after her adoptive mother was killed during the escape. The Pegasus sacrificed itself to save the crippled Galactica, but the crew survived, joining Galactica.
The Cylons then adopted the colonials’ mission to find the home of the Thirteenth Tribe, a planet they called Earth, intending to settle there. They resumed pursuit of the fleet, but upon reaching the Lion's Head Nebula, dispatched a Basestar to investigate. The Basestar took on board a canister left by the Thirteenth Tribe. The canister contained an airborne virus that proved deadly to the Cylons. The virus persisted through the download process so the Basestar that had been dispatched for the investigation was abandoned to avoid contamination. The colonial fleet discovered the Basestar and captured the ailing Cylons. The colonial fleet's attempt to use the virus to wipe out the Cylons was defeated when Helo, repulsed by the strategy, had the captive Cylons killed while out of range of a resurrection ship.
Meanwhile, Boomer turned increasingly anti-human. She was charged with Hera's care, but Hera rejected her. During a truce negotiation, Boomer told Athena that her daughter was alive but sick on the Baseship. She invited Athena to come to the Baseship and rejoin her people, because the occupation showed that humans and Cylons were incompatible and that humans would never truly accept her. Later, desperate for her child, Athena convinced Helo to kill her so she could resurrect on the Baseship. Once there, she retrieved Hera with the help of Caprica Six who killed a hate-filled Boomer to prevent her from killing the child. She had tried to prevent Athena from taking Hera to the humans and declared that Cylons were never meant to have children. Caprica Six returned with Athena and was detained by Adama in Athena's old cell.
Cylon struggles
As the pursuit continued, internal relations among the Cylons began to break down. A Three violated her programming by actively seeking information about the Final Five, seeing a vision of them on the Algae Planet where the Colonials stopped to get food. From that she was able to identify them. She and her "twins" were then "boxed". Within the fleet a musical activation signal (a version of All Along the Watchtower) was received by the Final Five, awakening their memories and bringing them together. Two were Galactica crew members (Saul Tigh and Galen Tyrol), while Samuel Anders was an athlete and Tory Foster a political operative. They kept their discovery secret. Tigh and Anders remained loyal to the humans, willing to fight and possibly die for them. Tyrol and Foster were ambivalent, with Tyrol having pro-human leanings. Foster became increasingly anti-human, murdering Tyrol's wife without remorse. After their realization, the fleet came under attack but, during the battle, a Cylon Raider identified Anders and he and his fellow Raiders broke off the attack, refusing to fight.
This set off a debate in the Cylon government regarding the Raiders. Ones, Fours and Fives voted to "lobotomize" the Raiders to make them acquiescent. Twos, Sixes and Eights, led principally by a Six named Natalie, became convinced that the Raiders had sensed the presence of the Final Five. Boomer’s assenting vote broke the tie. Natalie objected and removed inhibitor circuitry from the Centurions, granting them free will and independent thought. The Centurions then killed the models who were “lobotomizing” the Raiders.
In response, Cavil lured Natalie and her forces into a trap beyond resurrection range and slaughtered them. Only Natalie's crippled Basestar survived. With her Raiders destroyed except for some Heavy Raiders and the surviving Twos, Sixes and Eights (minus Boomer), Natalie allowed Two Leoben Conoy to locate Kara Thrace and ally with the humans. A deal was struck: the humans would help the rebels unbox the Threes and the rebels would lead the humans to the Resurrection Hub and help them destroy it so Cavil would lose resurrection forever. The humans transferred half of their Vipers to the Rebel Basestar. Fearing for Hera, Athena murdered Natalie, causing the Hybrid to jump the Rebel Basestar, with Roslin on board, away before everyone was ready. The Hybrid took the Basestar to the Hub where the allies had the human Vipers towed into battle by Cylon Heavy Raiders so they could catch the Cylons by surprise. The plan worked and the Hub's FTL drive was taken out, stranding it. While the Rebel Basestar engaged two other Basestars, Helo and an Eight boarded the Hub to unbox D'Anna and found her already unboxed by Cavil and Boomer. The three escaped and the Vipers nuked the Hub, destroying it and Cylon resurrection capability.
After a standoff, the identities of the four Cylons in the human fleet were revealed and together they found the devastated Earth, a nuclear wasteland. There, Tigh, Anders, Foster and Tyrol got flashes of their life on Earth and Tigh learned the identity of the fifth member: his dead wife Ellen. The humans and rebel Cylons (minus D'Anna), abandoned Earth in search of a new home together. The humans and Cylons ended up striking a deal: in return for joining the fleet as full members the rebels would upgrade the fleet's FTL drives with Cylon technology, increasing their jump distance by at least three times. This led to a political coup and failed mutiny, but left Anders seriously injured, although with his true memories returned. After telling the others as much as he could, Anders slipped into a coma from which he partially emerged while acting as Galactica's hybrid.
Cavil revealed that he had a resurrected Ellen Tigh as his prisoner, ever since her download after she was killed on New Caprica. Her resurrection restored her true memories, although her only company was Cavil and Boomer, whom he'd let in on the secret. After Cavil planned to dissect her brain to try to regain resurrection technology, Boomer helped Ellen escape to the fleet as part of a plan to kidnap Hera so the Cylons could study her.
Galactica and some of the rebel Cylons assaulted the Cylon space station that was the Cylon homeworld, to rescue Hera while the Rebel Basestar protected the fleet. During the battle, Boomer, who had begun to feel remorse for kidnapping Hera, killed the Four that was starting to dissect the child and carried her back to her parents. Athena then killed Boomer. The rescue team returned to Galactica where Cavil led a Centurion assault, but was defeated. He took Hera hostage, but was convinced to create a permanent peace with the humans in exchange for the Final Five giving him resurrection technology.
Tyrol learns that Foster murdered Cally and breaks the download, causing the Cylons to attack again. Cavil kills himself. An unmanned Raptor is struck by an asteroid, causing the dead pilot (Racetrack) to fire eight nuclear missiles into the Colony, knocking it out of orbit and into a black hole, destroying it and every remaining One, Four and Five. Galactica jumps away and arrives at a habitable planet. The humans and rebel Cylons settle there while the rebel Centurions are given their freedom and the Rebel Basestar to find their own destiny. Anders flies the human fleet and all its technology into the Sun. 150,000 years later, it is revealed that the planet they settled on is our Earth, while Hera was Mitochondrial Eve, which means all modern humans are partly descended from the humanoid Cylons.
Centurions
The Cylon Centurion is a mechanical infantry model, made less intelligent than human models. BSGr Centurions retain the silver appearance, robotic body, helmet-like head and oscillating red bar eye of their forbearers but are larger, taller, stronger, more agile and have a more streamlined appearance. They also have retractable guns built into their lower arms, bladed fingertips and heavy armour.
The Centurions were monotheists who believed in a single loving God.
Initial models
The first colonial Centurion is seen and is identified as a 'Cybernetic Lifeform Node'. Built on contract for the Caprican Defense Ministry, the Cylons are to replace human warriors on the battlefield (Caprica pilot).
The U-87 prototype Cylon contains a copy of the consciousness of Daniel Graystone's daughter, Zoe Graystone. Daniel Graystone conceives of the Cylons as a slave race and demonstrates this by instructing the prototype to rip her own arm off. After catching it interacting with the family dog, he realises that the prototype contains Zoe's consciousness and attempts without success to force her to admit her identity. She is able to defeat the tests he attempts, and convinces him that she is just a robot. Upon learning that Graystone is about to erase her memory, she escapes.
The Battlestar Galactica miniseries displays an original model Centurion on display in Galactica's museum hangar. The original Centurion is also depicted in a diagram reviewed by the armistice officer in the opening scene.
In Battlestar Galactica: Razor, Centurions are almost identical to those from the original series, except they are rendered using CGI and have exposed joints. The Centurion in the museum hangar is retconned as a CGI version with exposed joints. Initial-model Centurions are among those seen defending the Cylon colony from an assault by Colonial and rebel Cylon forces (Part 2 of "Daybreak").
The body had synthetic skin, although this model was still primitive and was mechanical rather than biological.[7]
Later model
Centurions do not download into new bodies when they are destroyed, according to screenwriter Ronald D. Moore The Hybrids have something to do with the programming of the Centurions. When the virus infected the Hybrid on the infected Basestar, the Centurions shut down ("Torn"). However, Centurions can function independently when no Basestars or skinjobs are nearby. A Centurion immediately fires upon an Eight when she unplugs a Hybrid ("Faith"). It is unclear whether this was in willful defiance of the Hybrid or as a result of the removal of their higher function inhibitor modules ("Six of One").
After the inhibitor modules that restrict higher functions, granting them independent thought were removed, although still largely servile, the Centurions were shown to exhibit human-like behaviour, such as politeness and consideration (e.g., responding to "please").
These Centurions are taller than their predecessors, and are sleeker. They possess less of a humanoid structure, averaging a head taller than humans. They retain the bipedal structure of the first model, as well as rough hands and feet. The feet of the new Centurions are a simple two-toed form that produces a distinctive sound when they walk as the toes move and touch the ground. They retain the distinctive, oscillating red "eye", as well as a rudimentary mouth (even though they cannot speak). The hands are much thinner and visibly segmented, and more claw-like (they can cut flesh and mortally wound a human,[8] making them effective close-combat weapons.) These claws can be modified at will, to a more 'finger-like' digit or less of a razor-sharp weapon.
Perhaps the most distinctive feature of this model is the twin cannons built into each hand, mounted on the side, that can be deployed by retracting the hands. It was originally established that the Centurions could not move while firing their cannons, however in later seasons they were seen to walk and even run while firing. The Centurions carry heavy weapons, such as anti-aircraft rockets that are strapped to their backs, for missions where their cannons are not enough.[9] Most Centurions can be destroyed or at least damaged by small-arms fire, however most require substantial artillery or special rounds to be destroyed (a Centurions party that boarded Galactica were especially strong, and required explosive rounds to be destroyed, as regular rounds proved completely ineffective[8]).
These Centurions greatly outmatched the previous model.[10] They were better-armoured, stronger, and carried internal weapons (as opposed to their predecessors, who needed to carry weapons such as rifles and pistols, like their human creators).
Raiders

The later form of Cylon Raiders are biomechanical entities integrated into small fighter spacecraft. They resurrect when they die. Raiders are subservient, and were compared to trained animals by the humanoid Cylons. A Raider nicknamed "Scar" developed a personality ("Scar"). Scar was the Cylons' top gun, driven by a bitter hatred of humans. He was killed and reborn many times. Raiders do not use vocal communication, appearing to communicate through unspecified electronic signals. Modern Raiders, unlike their predecessors in the First Cylon War, are alive, with a complex system of veins, organs and biological fluids contained within their main body.[11] The Raiders are programmed, but began with some measure of autonomy and personality. When one Raider saw a Final Five in the human fleet during an attack,[12] it retreated, leading the remaining Raiders back to the Basestar. The Raiders then refused to fight the humans, for fear of harming their Cylon kin. This led to the lobotomy vote.
Well-trained Colonial pilots, even nuggets, can beat a Raider in most situations. The Raiders' primary advantage seemed to be in their massive numbers: while Galactica carries around 40 Vipers, Cylon Basestars support 300-600 Raiders. The Raiders are maneuverable, once described by Kara as a "squirmy son of a bitch", and most human pilots were unable to keep them in sight for more than a few seconds.[13]
Weapons
The Raiders' primary weapons are two kinetic energy guns mounted on the underside of their wings, close to the main body. They may carry conventional missiles and tactical nuclear weapons. Raiders, by revealing their red "eye", can transmit deadly computer viruses or signals to enemy ships, which strike with brutal efficiency and can shut down Colonial vessels, turning off their power and making them easy prey. If all else fails, Raiders sometimes attempt suicide attacks. A single Raider can destroy a sizeable transport.[12]
The Raiders from the First Cylon War were larger spacecraft driven by Centurion pilots, and not alive. These are used by the remaining First War Centurions guarding the Original Hybrid and the Cylon Colony, and later became obsolete.
Heavy Raiders
The later Cylon Raiders are different from Cylon Heavy Raiders. Like the raiders of the First Cylon War, the Heavy Raiders are transports and attack bombers, and are primarily piloted by Sixes and Eights.
Humanoid cylons
Although they made only a single appearance in the original continuity (the episode "The Night the Cylons Landed" from Galactica 1980) humanoid Cylons are the focus of the reimagining. They are usually known as "the Cylons", or derogatorily as "skinjobs" or "toasters". Skinjobs are indistinguishable from humans due to their creation through synthetic biology, employing real bones, flesh and blood rather than metal, but still possess a "digital molecular structure". Some are programmed to believe that they are "organic" until otherwise informed.
The Cylons can communicate nonverbally, but do not have a hive mind, even among a single model, and are most often shown talking to one another.
The thirteen distinct humanoid models are divided into the "Final Five" that originated on Earth, and "Significant Seven" (the latter term is only used by the Battlestar Galactica producers). The Final Five are unique, and are the last survivors of the Thirteenth Tribe. They developed the Seven (originally eight) in conjunction with the Centurions of the Twelve Colonies. Each copy shares biology and general personality throughout their model, but they develop into distinct individuals. The Final Five showed the Centurions how to make skinjobs and attempted to give them human characteristics (love, mercy), thinking that these characteristics would prevent war.  
Prior to the "Final Five" arriving to stop the first Cylon War on Caprica, the Centurions had already made their first humanoid, called the Hybrid, for the purpose of controlling the baseship.  Once the Final Five arrived on Caprica, they transferred their technology to the Centurions and gave them eight fully humanoid models created by them.
Cylon society is made up of the seven, whose types are numbered One, Two, Three, Four, Five, Six and Eight. The extinct Seven model (the "Daniel" series) was destroyed by a One decades prior to the Colonies' destruction.
Origin

As revealed towards the end of season 4, there is a 2000 year gap between the exodus of the first humanoid Cylons (the Final Five) from the destruction of life on Earth due to a robot Centurion war there, and their arrival at Caprica in the middle of the Caprican robot Centurion war.  Originally, the Final Five had left Earth both to seek a new home and to warn the other Twelve Colonies about the dangers of mistreating Centurions so that they might avoid the fate of Earth. However, at the time FTL (faster than light) starship jumping technology was unknown, so they made the journey using slower than light technology, resurrecting themselves when their bodies wore out over the span of 2000 years.  
It should be noted that when the Earth was nuked, 2000 years prior, the Final Five Cylons had already planned for this event and were resurrected onto their baseship in orbit when their bodies were destroyed in a global nuclear attack that wiped out all life on Earth.  This resurrection was the event which triggered the Final Five Cylons to begin their 2000 year journey to Caprica, only to find out upon arrival that humans and Centurion robots had begun fighting towards extinction just as they had 2000 years prior on Earth.
The Final Five arrived at Caprica to discover that the Caprican Centurions had already begun experimenting with creating humanoid Cylons by creating the first Hybrid to control their baseship.  As part of the agreement to end the war with humans, the Final Five Cylons gave the Caprican Centurions eight humanoid Cylon models and a resurrection ship.  
During the 40-year gap between the end of the first Cylon war and the second Cylon war on Caprica, Cavil, model number One of the eight humanoid Cylons that the Final Five had designed and created, rebelled against the Final Five and took command of the Centurions and of the other humanoid Cylons.  In order to get rid of the Final Five who blocked him from continuing the Cylon war, he tricked the Final Five into to living on Caprica with their memories erased and false memories implanted.  He was deluded into believing that he could show the Final Five that they were wrong about humans by letting them suffer with the humans as their civilization was destroyed (as a result of him restarting the Cylon war) and then restoring their memories while resurrecting them on the Cylon baseship. Cavil thought this lesson would help the Final Five realize that humans were flawed creatures and thus persuade them to take his side.
Ellen, the original creator of the eight humanoid models, programmed them with a belief in a single God that is all-loving.  She did this because she wanted the Cylons to know love and to be peaceful and to avoid war in the future.  However, Cavil rejected his programming of believing in an all-loving God. Other Cylons, particularly the D'Anna model, justified attacking the humans perhaps as a form of religious fundamentalism, with the goal of remaking the world as a Cylon One-God utopia.
In addition, Cavil regarded the creation of humanoid Cylons as a mistake and believed that Cylons should return to a purely mechanical form so that they could experience the world in ways not limited by human senses. For example, he said that when a star explodes into a supernova, he wants to see the X-rays and gamma rays with his eyes, and not be limited by biological human eyes.  
Cavil was fearful of the mechanical Centurions, rejecting the secret removal of circuitry to prevent the Centurions from having free will in Season 4. When Centurions on one of the Cylon baseships received this modification and thus developed free will, they revolted against Cavil because they rejected his policy of removing the biological brains of the Raider ships and returning them to a purely mechanical form so that they would follow orders without emotion interfering. This small rebellion left the Sixes and Eights in charge of one of the baseships in the Cylon fleet. The rebel baseship later forms an alliance with the human fleet after being attacked by Cavil's Cylon fleet and left to drift in space without FTL propulsion.
Roles
The skinjobs are responsible for command decisions. While the command structure has not been fully explored, some hints at organization are known. At the end of the mini-series, a group of skinjobs debate what to do, and one speaks authoritatively, to which another responds, "by your command", a reference to the original series. They can freely impose blocks on the behavior and cognitive programming of Centurions, Raiders and Hybrids. Multiple skinjobs with blocked memories are spread among the surviving humans, including members of Galactica</i>'s crew.
A Cylon council (made up of multiple copies of several models) appears to cast votes collectively by model, indicating that model plays an important role in the structure of Cylon society ("Precipice"). Some models are implied to have higher or lower ranks than others. During the occupation of New Caprica, a select group of individuals, who mostly had been within the Colonies, such as Boomer and Caprica Six, seemed to have seniority within their model. One of each model was seen deliberating and discussing their course of action, with each of the 'senior' copies representing the opinion of their model.
Although little is known about Cylon psychology, it is clear that they share many human emotional vulnerabilities. This is evidenced by Gina's catatonic state as a result of the severe trauma of her abuse at the hands of the Pegasus crew. This abuse was presented in the form of multiple sexual assaults, battery, starvation and outright torture. Six/Caprica Six also revealed that Cylons exist by 'projecting' an environment of their choice on the real/physical world.
Number One (John Cavil)

Ones subvert consensus by undertaking many actions without the knowledge or consent of their siblings, such as turning against the Sevens and the Final Five, and then reprogramming their siblings to hide this.
The main antagonist of the series, John Cavil, initially poses as a priest ("Lay Down Your Burdens"). His true identity is revealed when a second copy boards Galactica and reveals his Cylon nature. Cavil's role in society is contradictory. He occasionally serves as a devil's advocate, pointing out the absurdity of his fellow Cylons and their religious zealotry. He was revealed to be a One ("Six of One").
Cavil has a sadistic, Machiavellian streak that none of the other Cylon models share. He tortured Saul Tigh by gouging out one of his eyes and showing it to him. He took pleasure in blackmailing Ellen Tigh into providing sexual favours in exchange for releasing Saul from captivity on New Caprica.
Cavil is an atheist (alone among the seven models), and often mockingly uses air quotes when saying the word "God". His opinions of humans are contradictory. He is one of the most violently anti-human Cylon models, advocating a policy of culling humanity down to a "controllable number". He claimed to have been against the destruction of the Twelve Colonies and advocates a unique Cylon Society, not one that emulates its creators. After arranging Tigh's release, Cavil further manipulated the situation by again blackmailing Ellen to betray the human resistance on New Caprica and threatening to kill Saul if she did not comply.
Cavil is aloof compared to the other models, though Boomer implies that Cavil has begun teaching her his views that Cylons should not emulate humanity. Cavil knows aspects of Cylon lore that the others models do not. His knowledge of the Final Five led him to box the D'Anna and the other Threes to ensure that whatever she had learned about the Final Five stayed hidden.
He was made in Ellen's father's image. Ellen also named him "John". He hates this name and shows deep resentment towards Ellen at being 'limited' by having to live in a humanoid body.
One was the first model and helped the Final Five create the other seven. Therefore, Ones knew about the Final Five while the others knew only that they existed. For an unstated reason, Cavil rejected the human trait of mercy and turned against the Final Five.
When the Final Five were resurrected, Cavil implants new memories for their life in the Twelve Colonies. He also alters the original programming of the seven other models so that searching for, talking about, and even thinking about the Final Five was strictly forbidden.
Ones are played by Dean Stockwell.
Number Two (Leoben Conoy)

Leoben first appears as a smuggler at the munitions depot at Ragnar Anchorage, where he was exposed as a Cylon (mini-series). Twos are religious zealots, carrying a pathological, and sometimes sexual, obsession with Starbuck. His Cylon identity was revealed ("Six of One").
Twos are skilled at deceit. Conoy manipulates Starbuck into thinking that a young girl he had kidnapped on New Caprica is her biological daughter. Twos display cunning minds and the ability to find and exploit others' weaknesses. Unlike other Cylons, Twos appear to be prescient and on several occasions prophesy future events.
Twos are played by Callum Keith Rennie.
Number Three (D'Anna Biers)

D'Anna Biers was a reporter for the Fleet News Service. Roslin and Adama grant Biers access to all areas of Galactica to investigate her suspicions of a military cover-up surrounding civilian deaths aboard the Gideon. Their belief is that Biers will discover that military personnel deal with the same pressures and fears as the rest of the fleet, and that the events were a tragic mistake. The Biers model used the documentary as a means to relay information that Sharon was alive to Cylons on Caprica ("Final Cut"). D'Anna was revealed to be a Three in the episode "Downloaded".
Threes are among the more aggressive models, and share a fatalistic understanding of the Cylon religion, believing that everything that happens is the will of God. Their religious devotion masks a deep questioning by Threes about why God allows bad things to happen (Theodicy).
Threes possess an "alpha-female" aggressiveness, advancing themselves as de facto leaders in any situation. They are often rivals of Ones and Sixes. The rivalry with the Sixes partly stems from when Caprica-Six killed a Three to save Samuel Anders from torture and death. D'Anna and Caprica-Six later reconcile and both of them are involved in a relationship with Baltar. Baltar seduces D'Anna by exploring her crisis of faith, which leads to her secretly committing suicide (a major Cylon taboo) to experience resurrection. She feels these near-death experiences will answer questions regarding her faith, notably the identity of the Final Five. D'Anna eventually sees the faces of the Final Five ("Rapture"). Unfortunately, as she does so, the other Cylons vote to box all Threes. Cavil tells her that what she had seen regarding the Final Five was unacceptable and that her memories would be kept in "cold storage". D'Anna, but not the other Threes, is later unboxed ("The Hub").
Threes are among the more anti-human Cylons. D'Anna tells Baltar that she believes that humanity must die to prevent future generations of humans from seeking revenge ("Exodus" Part II). She is temporarily the leader of the "rebel" faction of Cylons that makes peace with the human fleet, but when the first Earth is found to be a devastated ruin, she decides to stay there and die rather than continuing the cycle of death, exodus and rebirth.
Threes are played by Lucy Lawless.
Number Four (Simon O'Neill)

O'Neill poses as a human physician who treats Starbuck for a gunshot wound in what was supposedly a resistance hospital on Caprica ("The Farm"). In reality this was a Cylon "farm" set up to breed human-Cylon hybrids using captured humans. Simon performed invasive tests on Starbuck's reproductive organs. He was revealed to be the 4th model in the episode "Six of One". Fours are consistently medical specialists.
Another Four poses as a married man in the Colonial Fleet. Cavil at one point asks Simon to leave his life behind and destroy the ship he lives on. He defies this order to protect his wife and her child by committing suicide and flying out of an air lock. He was not close enough to a Resurrection Ship and therefore died permanently ("The Plan").
Fours are played by Rick Worthy.
Number Five (Aaron Doral)

Doral debuts as a public relations worker for the abortive Galactica museum (miniseries). Another Five sneaks aboard the Galactica and detonates an explosive ("Litmus"). This event prompts the fleet-wide announcement that Cylons can have a human appearance. He was revealed to be the 5th model in the episode "Downloaded."
During the Occupation of New Caprica, the Fives are anti-human and aggressive. They specialize in public relations and control, using large-scale events and media manipulation.
Fives are played by Matthew Bennett.
Number Six (Caprica-Six / Shelly Godfrey / Gina Inviere / Natalie Faust / Lida / Sonja)

The first model explicitly revealed to be a Cylon in the mini-series, Sixes often use seduction to their advantage. Important copies include Shelly Godfrey, Gina Inviere, Head Six, Caprica Six and Natalie.
Caprica Six
One of the Sixes, later known as Caprica Six, on her way to meet with Baltar, murders an infant by breaking its neck. This could either be considered sociopathic or a mercy killing due to the coming Cylon attack. She saves Baltar's life when his house was struck by a blast wave from the Cylon attack.
Caprica Six has a love-hate relationship with Baltar, alternately defending him to other Cylon models or leaving him to suffer at their hands.
A reincarnation of Caprica Six commits the first incident of Cylon-on-Cylon violence, killing a Three with a rock to save Ander's life.
Caprica Six was imprisoned on Galactica after helping Athena escape from the Cylons with Hera. She offers to testify against Baltar. While in the brig, she has a relationship with Saul Tigh that results in the first Cylon-Cylon pregnancy in 2,000 years, which ends in a miscarriage.
Caprica Six has an "Inner Baltar" which only she sees, similar to how Baltar has an "Inner Six" that only he can see; Inner Baltar serves as a sort of conscience for Caprica Six and as an advisor in times of trouble, as Inner Six does for him. Inner Baltar appears at least once to Baltar himself. Before their true nature is revealed, speculation rises that Baltar is a Cylon. Inner Baltar and Inner Six appear to Baltar and Caprica Six – all four sharing a moment together – as the first part of the revelation that the "Inners" are divine beings (referred to as "Angels") manipulating both the Colonials and Cylons into fulfilling "God's plan" (series finale).
Shelly Godfrey
Godfrey was on Galactica after the Cylon attack. She accused Baltar of treasonously sabotaging human defenses, and therefore of killing several million people (Six Degrees of Separation). She also appears in Battlestar Galactica: The Plan TV movie.
Gina Inviere
Gina acted as a contractor for Pegasus and was in a relationship with Admiral Cain. When Cain discovered that Gina was a Cylon agent, she ordered her crew to "push her programming" in an attempt to see if Gina was vulnerable to emotional trauma. Gina later escaped with Baltar's aid, assassinated Cain and founded a resistance group in the fleet, preaching peace with the Cylons. While it seemed that Gina developed feelings for Baltar and wanted to pursue him, she detonated a nuclear warhead (provided by Baltar) that destroyed Cloud Nine, other fleet ships and generated an electromagnetic signature, the detection of which ultimately led the Cylon fleet to New Caprica.
Sonja and Natalie
Other important Sixes include Natalie, who originally leads the rebel Cylon faction against Cavil's faction before she is killed by Athena in a misunderstanding, and Sonja, who was elected to represent the rebel Cylons in the Colonial fleet's Quorum of Ships' Captains.
Sixes are played by Tricia Helfer.
Number Seven (Daniel)
Daniel was the seventh Cylon model ("No Exit"). Ellen refers to Daniel as an "artist, and so sensitive to the world". She was very close to Daniel, which enraged Cavil; he felt that Daniel was Ellen's favorite and became jealous. Cavil poisoned the amniotic fluid used to mature the Daniel copies and then corrupted Daniel's genetic code. This sabotage meant that no further Sevens could be created. The original was also killed with Samuel Anders stating at one point that "Daniel died" after regaining his true memories. 
Fans speculated that Starbuck or her father was a version of the corrupted Seven line. Instead Daniel is merely a plot device to explain the missing number and to expand on Cavil's character (noting that Daniel is essentially the Abel to Cavil's Cain).[14] The Sevens were a springboard for Caprica, where Daniel Graystone is the name of a main character played by Eric Stoltz; this Daniel is the creator of the Colonial Centurions.[15]
Sevens are never seen in the series, and their physical appearance is never described.
Number Eight (Sharon "Boomer" Valerii / Sharon "Athena" Agathon)
At the end of the miniseries, a Six says "By your command" to an Eight. This phrase is usually spoken to the Imperious leader in the old series by Centurions. It seemed to give this particular Eight a leadership role.
Boomer
Sharon "Boomer" Valerii first appears as a Raptor pilot aboard the Galactica. Her true nature as a Cylon was revealed at the end of the miniseries. She acted as a sleeper agent, unaware of her true nature and programmed to carry out attacks on the fleet without realizing what she is doing. She was revealed to be an Eight in the episode "Downloaded".
Athena
The other significant Eight, Sharon "Athena" Agathon, married Karl "Helo" Agathon, rebelled against the Cylons and joined forces with the Colonials. Her child with Helo, a daughter named Hera, is the first Cylon/human hybrid. Such a child is the subject of a Cylon prophecy. Its importance increased as it was a 'child born of love'. Shortly after Hera's birth, Roslin decides to mislead the Cylons into believing the child has died. Her plan involves lying to Athena and Helo, and giving the child to an adoptive mother, Maya. While aiding the Colonials in their plan to rescue the humans on New Caprica, Athena sneaks into a Cylon storage facility and steals launch codes for the civilian ships trapped on New Caprica. She is discovered by D'Anna, who informs her that Hera is still alive, citing strange dreams and a prophecy by a human mystic as proof. Athena, believing this to be a ruse, kneecaps D'Anna. During the exodus from New Caprica, Hera's caregiver is killed and D'Anna finds the baby. Boomer later tells Athena that Hera lives. Athena has Helo kill her (Athena), allowing her to download into a new body within the Cylon fleet. With the help of Caprica Six, she rescues Hera and returns to Galactica.
Eight is described as "weak" by Baltar's Head Six, and Eights usually appear more compassionate and sympathetic than other Cylons. However, they fully supported the destruction of the Colonies. The actions of Boomer and an Eight who had a duplicitous affair with Felix Gaeta on New Caprica make it clear that Eights are capable of homicide and betrayal. Eights are also capable of intense loyalty and have the ability to break from Cylon traditions and laws to help human friends or family. They vote to save humanity in the Cylon civil war that Boomer starts. She hesitates for a while when Cavil influences her, but in the end chooses to support the humans, even if it means that she must give up her life.
Athena becomes completely assimilated in human culture. Her child and husband become her life's focus. She is the only Cylon to create a family. The Eight's perceived fragility camouflage great strength and direction. In one episode, Athena hardwires herself to Galactica, hacks into a Cylon fleet and shuts them down.
Eights are played by Grace Park.
Final Five 
Cavil killed and boxed the Final Five. Later he resurrected them, purged their memories and sent them to live among the humans. The other six models had no knowledge of the Final Five's identities, presumably blocked by Cavil, though they were aware of the existence of five absent models who were not to be spoken of. When D'Anna (a Three) attempts to learn their identities, Cavil boxes her entire model line as punishment. In the final episode of the third season, four of the final five are revealed to be characters with long histories on the Galactica. Their purpose, and how and why they were hidden from the rest of their kind, is a major plot point of the fourth and final season when Ellen Tigh is revealed to be the fifth member.
Backstory
The Final Five were Cylon models who could not be named or researched by skinjobs ("Rapture"). They were the original humanoid Cylons, born 2,000 years before the series, on Earth, as part of the Thirteenth Tribe. ("Sometimes a Great Notion") They were born to Cylon parents through sexual reproduction, rather than built. They were the researchers who rebuilt the Cylon resurrection technology that had been abandoned when the Thirteenth Tribe gained the ability to procreate. Saul and Ellen were married at the time, while Tyrol and Foster were in love and planned to marry ("No Exit").
While on Earth the Final Five learned of the upcoming attack on the Colonies from beings ("Angels") only they can see. They placed the resurrection technology on a ship in Earth orbit. When the nuclear attack kills them, they resurrect on the ship and head for the Colonies to warn them. They arrive during the First Cylon War, unbeknownst to the humans, and make a deal with the Centurions: stop the war and they will help them build human bodies. They build the first model, One (Cavil), who helps them build seven other models ("Sometimes a Great Notion").
Cavil then kills and boxes the Five and removes the memory of their identities from the other Cylons. He later unboxes them, replacing their memories. Cavil periodically seeds them among the human populations starting with Saul, and then Ellen, to show them the evils of humanity.[16]
The Five are fully Cylon – although "fundamentally different" from the others.[17] Unlike the other models, they do not have model numbers.[18][19]
Other Cylons do not talk about the Five ("Torn").
Identities
Saul Tigh (Michael Hogan) and Galen Tyrol (Aaron Douglas) are Galactica officers, while Samuel T. Anders (Michael Trucco) is an athlete on Caprica. Tory Foster (Rekha Sharma) is a political operative who is rescued from Caprica and works for Roslin. Ellen Tigh (Kate Vernon), is the fifth member, as Saul later realizes as more of his memory returns. ("Sometimes a Great Notion") Ellen resurrected after she is poisoned by Saul for giving rescue mission plan to Cavil on New Caprica. This restores her memory of her life as a Cylon ("No Exit").
Story
The Five awaken when they begin to hear strange music. This seems to affect other Cylons. A Raider scans Anders and disengages an attack. Aboard Galactica, Caprica Six tells Roslin that she can feel the Final Five and that they are near. ("He That Believeth In Me") Twos, Sixes and Eights believe that the Raiders called off the attack because they sensed the Final Five, something with which the Ones, Fours and Fives disagree. This disagreement leads directly to the Cylon Civil War, in which the Twos, Sixes, and Eights rebel ("Six of One"). A standoff between Galactica and the rebel Baseship exposes the four as Cylons to the humans, but Lee Adama, in his capacity as acting President, grants them amnesty as part of an alliance ("Revelations").
Only Anders and Ellen regain their complete memories: when Anders gets shot in the head, it seems to break Cavil's block on his memory, while Ellen's resurrection restores all of her memories. Cavil had planned for the Five to die in the destruction of the Colonies, download, regain their true memories and apologize for their faith in humanity (The Plan). Instead, four of the Final Five survive the destruction of the Colonies without resurrecting (Tigh and Tyrol were on Galactica and Tory and Anders survived through luck) while Cavil kept Ellen alive so she could suffer more and learn her "lesson". Cavils' plan thus fails, as all of the Five (with the possible exception of Foster) maintain their loyalty to humanity.
They play a major role in ending the second war, taking a prominent role in the Battle of The Colony, especially Anders who acts as Galactica's Hybrid and shuts down the Colony's weapons and Hybrids. The Five also nearly bring a peace between human and Cylon by exchanging resurrection technology (which can only be rebuilt by the combined knowledge of the Five) for peace.
They see each other's memories and Tyrol learns that Tory had murdered his wife. In a rage, he strangles her, shattering the temporary peace, although a fortuitous nuclear explosion and Cavil's subsequent suicide effectively ends the war moments later.
After escaping and reaching a new Earth, Anders flies the fleet into the Sun, destroying it and himself, leaving only three of the Final Five alive.
Hybrids

The Hybrids resemble human beings inside an immersion tank similar to a Cylon rebirthing tank. The Hybrids are cyborgs, consisting of conduits and other connectors mated to biological elements. The Hybrid is not one of the "thirteen models". It represents a step on the path from mechanical Centurion, to partially bio-mechanical Raider, to Hybrid, to skinjob. Hybrids are so integrated into the basestar's functionality that they are, for all practical purposes, its brains.
Hybrids continually speak what most of the humanoid Cylons consider to be gibberish, although there is some difference of opinion on this point. Many Cylons believe a hybrid's conscious mind is completely mad and the functions it performs are part of a deeper state of mind in connection with the ship. Caprica Six states that the Conoy model believes that every word a hybrid says is channeled from the Cylon god. A Hybrid is not allowed to vote in the Cylon democratic process, though it sometimes objects to the resulting decisions, for example, leaving another basestar full of disease-infected Cylons to their fates.[20]
One Hybrid, considered the First Hybrid in Cylon lore, was the result of experiments on humans during the First Cylon War and appears to be more coherent and prophetic than the others. It went rogue as early as the end of the First War, and is protected by model 0005 Cylons called Guardians, who consider it their god. According to the skinjobs, the First Hybrid and its Guardians are legends. The Colonial Fleet accidentally clashes with the Guardians, destroying them, the First Hybrid along with its basestar. The First Hybrid has the appearance of an old man, whereas the standard Hybrids look like young women and are identical.
After he is shot in the head and enters a vegetative state, Samuel Anders is connected to a datastream and as a result develops Hybrid-like abilities, whereupon he acts as Galactica</i>'s Hybrid. When hooked up, he speaks and acts like a Hybrid. This shows that apparently under special circumstances, normal Cylons, or at least the Final Five, can become Hybrid-like beings.
The Basestar Hybrids are played by Tiffany Lyndall-Knight. The First Hybrid is played by Campbell Lane.
Religion
Humanoid Cylons, except for the Cavil models, follow a monotheistic religion. Religious fanaticism partially motivates their attempted genocide of humanity. Despite their origins the Cylons believe themselves to be spiritual beings. This monotheism seems to share some of the characteristics as the Abrahamic religions: belief that God is omniscient, omnipotent, omnibenevolent, that he will one day deliver divine retribution and that he intervenes in the mundane world. Their beliefs are different from the Abrahamic religions in that they deal with issues such as consciousness, enlightenment and reincarnation.
Moore said that this spirituality comes when a species becomes sentient and self-aware – questioning faith and religion and about what happens after death.[21]
The different Cylon models have slightly different ways of talking about and dealing with their god, reflecting the different aspects of humanity that each Cylon model reflects.[21] Ones do not all believe in God and can be agnostic or atheist. In contrast, Twos are fanatically religious.
Vulnerabilities
Some Cylons are physically stronger than humans. While this is obvious with respect to Centurions, the extent to which the Twelve are also is unclear. In the miniseries, William Adama fought with a Two who, despite suffering from the effects of radiation, proved to be stronger. The Two managed to break off a metal pipe to use as a club and lift Adama off the floor with one arm. Another Two was able to break apart a pair of metal handcuffs while under interrogation ("Flesh and Bone"). A Six engaged Kara Thrace hand-to-hand and proved to be exceptionally fast, agile and strong ("Kobol's Last Gleaming II"). Despite this Cylons are vulnerable to most of the same things as humans. The Two who fought Adama was beaten to death with a flashlight, while the Six who fought Thrace was impaled by a piece of rebar. While the Twelve may match or exceed human strength, they do not display superhuman powers or resilience. Cylon skin appears just as vulnerable to breaking as human skin, bleeding just as a comparable wound would in a human.
Cylons' mental faculties are sufficiently similar to those of a human to allow for manipulation. Cain and Thrace exploited this fact while 'interrogating' Cylons. The experience of death (even with resurrection) is traumatic and can leave deep emotional damage.
Cylons are susceptible to an ancient virus that can be transmitted by rodents to which humans are immune, lymphocytic encephalitis. This virus was carried on a beacon left by the Thirteenth Tribe at the Lion's Head Nebula millennia before and disrupts all organic Cylon technology including Raiders, Hybrids (which in turn disrupts Centurions) and the skinjobs. The humans speculate that the beacon was accidentally contaminated when someone sneezed. The Cylons believe that a critical symptom of this virus, a harmful bioelectric feedback, can be transmitted during a download and attempt to prevent infected Cylons from downloading. The only immune Cylon is Athena, who had given birth to a half-human child and thus 'inherited' human antibodies from her offspring (although it is unclear whether she retained this immunity after downloading).
Dr. Cottle developed a vaccine after some Twelves were infected on a basestar near the Lion's Head Nebula. The disease requires regular booster shots to keep an infected Cylon healthy. The Cylons failed to develop a cure or a vaccine to the virus.
Cylons' silica pathways are affected by certain forms of radiation, such as that surrounding Ragnar Anchorage. It is assumed but not stated that this radiation affects all of the skinjobs as the station was chosen as a refuge by Commander Adama before he learned of the existence of biomechanical models. The radiation had deleterious effects on the Two they found at Ragnar Anchorage but not on Adama. This suggests that humans are not vulnerable to this type of radiation.
Plutonium is used for Baltar's Cylon Detector because its radiation affects humans and Cylons differently. The detector was dropped as a method of exposing hidden Cylons early in Season 2 as the revelation that Boomer (a test subject) was a Cylon convinced the command staff that the detector did not work.
See also
Cylon vessels
Basestar – controlled by Hybrids
Raider – individuals in their own right
Heavy Raider
Resurrection ship




 at Battlestar Wiki
 at Battlestar Wiki