Jobs & party roles

There are many jobs and party roles in the game, and learning them all can be daunting. This short guide is intended to help make it clear what roles each job can fulfill, and what to expect from playing those jobs. Many jobs can fulfill multiple roles, so some may be listed multiple times. These are general rules of thumb (many players stretch or break these roles) but should give you an idea of where your job stands. I also suggest reading over other jobs and roles to get an idea of how they play (and maybe try some others out, if they sound fun!). 
Note that this is intended to be a very surface-level overview of the jobs, and is meant to only be an introduction to each of the jobs. If you'd like more information on each job, I suggest heading to their pages on this wiki, which will be linked for each job. There you can find guides dedicated to each job, which will give more detail that is out of the scope of this guide.
=Heavy Physical Damage Dealers (Heavy DD / DPS)=
These are the heavy hitters, as the name implies. They often wear heavy armor and use two-handed weapons. They can be surprisingly survivable, but don't offer much else in terms of party support beyond pure damage and the occasional debuff.
Warrior - The classic heavy DD, WAR is flexible in that it can wield many different weapons. Can function as a tank in a pinch. Good in almost any situation where you need physical damage. Uses almost all weapon types, but has a fondness for Great Axes
Monk - MNK doesn't wear heavy armor, but is very survivable nonetheless due to being able to counter enemy attacks. Can buff party max HP, which can help save the team during some boss attacks. Uses hand-to-hand weapons and kick attacks. Good for soloing.
Dark Knight - DRK arguably has the single highest damage Weapon Skills in the game, but offers little utility outside of that. Can cast some black magic as well! Uses Great Swords and Scythes. 
Samurai - SAM specializes in Skillchains, and can solo multiple Skillchains by using a ton of weapon skills in a row. Good for "zerging" down bosses due to this ability to pump out big burst damage. The only class that specializes in Great Katanas, but can also use Polearms and Bows.
Dragoon- A mix of DRK and SAM, with a pet wyvern thrown in - DRG deals good Weapon Skill damage and can solo Skillchains easily due to their Jump skills. You need to pay attention to keeping your pet wyvern alive, though you have lots of options for getting rid of hate and keeping yourself alive. Specializes in Polearms. Good for soloing with a WHM or BLU subjob.
Notes: With very good gear, Rune Fencer can also function as a heavy DPS.
=Light Physical DD=
These jobs wear light armor, and usually dual wield weapons. Don't let the name fool you, they can often dish out nearly as much damage as a Heavy DD (if not more, depending on gear) - however, they often lack the survivability of the heavier jobs. They tend to make up for this by offering more support with debuffs, buffs, and even healing. Many light DDs are also more suited than other jobs for solo / "low man" situations where you don't have a full party of other players, due to having a mix of strong damage and sustainability. 
Red Mage - A jack of all trades, RDM can deal some very solid damage with their swords, while dealing extra magic damage on each swing with their en-spells. They can solo shorter skillchains, and even magic burst off of them - all while offering buffs and healing. Primarily dual wields Swords and/or Daggers. Very good for soloing.
Thief - THFs help control hate / enmity of party members, while dealing out big burst damage with their position-based skills. They also bring Treasure Hunter to the table, helping with rare drops. Primarily dual wields Daggers, but occasionally swords. Good at soloing with a NIN or DNC subjob. 
Beastmaster - BSTs summon various helper pets to deal damage and help kill the enemy. They can deal very strong AOE damage, and are extraordinarily good at soloing. Primarily dual wields Axes, or an Axe and Shield.
Ranger - RNGs can deal very good damage both up close with melee attacks and at a distance. They lack party support options, but can deal quite a lot of damage. Dual Wields Daggers, Swords, and Axes for meleeing, and Bows, Crossbows, and Guns for ranged damage.
Ninja - NIN are similar to RDMs in that they can deal their own skillchains and magic burst off of them with ninjutsu. They're also very survivable by avoiding damage, and can even serve as a tank in some situations. They dual wield one-handed katanas and carry throwing shurikens.Good at soloing.
Summoner - SMNs summon up pets to do their dirty work, and they can deal very strong physical (and magic!) damage, along with a variety of buffs and debuffs. However, it can be difficult for a new player to gear up, as they require quite a lot of different equipment to become effective. They carry staves, but don't use them for meleeing, instead sending their summoned Avatars in to fight.Very good at soloing.
Blue Mage - BLU uses enemy skills to bring a lot of variety to the table, dealing very respectable melee damage along with lots of AOE debuffs + damage. Learning spells can be time consuming, though. They dual wield swords. Very good at soloing.
Corsair - A great party support with very strong (if random) buffs, COR can also deal respectable damage. In very high demand for parties. They dual wield swords and daggers for melee, and use Guns for ranged damage.
Puppetmaster - PUP is similar to BST in that they have a pet to deal damage for them. The puppet is very customizable, and can deal melee or ranged damage, and can tank surprisingly well. Puppetmasters wield hand-to-hand weapons, but generally let their puppets do most of the fighting. Good at soloing. 
Dancer - DNC specializes in skillchains just like SAM, but sacrifice some damage in exchange for very effective debuffs and healing. You'll need to use Sambas, Waltzes, Steps, and Jigs to manage a resource called Finishing Moves to deal big burst damage. They dual wield daggers.Very good at soloing.
Notes: At higher levels, Bard can also function as a light DPS with melee dagger attacks and weapon skills.
=Magic DD=
Magic DDs focus in magic damage - not very surprising. This is usually done at a range, and often can hit multiple enemies at once. Their damage is often very strong, but are generally very squishy and can die easily if they get too close. Most mages use Staves or clubs, primarily carrying them as boosts to magic stats. 
Black Mage - The gold standard of magic damage, BLM are all about nuking hordes of enemies down with big explosions. They provide some light debuffing options, but are always brought along to bring their amazing firepower - especially when Magic Bursting off a Skillchain. 
Red Mage - RDMs lack the AOE of a BLM, but can deal very respectable single-target magic damage while also bringing strong debuffs, buffs, and healing. Also able to deal lots of magic damage through melee attacks using enspells. Good at soloing.
Summoner - SMNs summon up pets to do their dirty work, and they can deal very strong magic (and physical!) damage, along with a variety of buffs and debuffs. However, it can be difficult for a new player to gear up, as they require quite a lot of different equipment to become effective. Very good at soloing.
Blue Mage - BLUs can dish out very strong AOE magical damage, often dealing status effects too. As mentioned above, they also perform well as a Light DD. Very good at soloing.
Scholar - Very flexible mages, SCHs flip-flop back and forth between specializing in white and black magic. Can deal very strong damage over time and can even be a primary healer, if you're good at juggling its Stratagem charges. It can even create skillchains via magic!
Geomancer - While primarily used as a support job, GEOs can bring solid magic damage to the table. In AOE form, too!
=Tank=
Tanks are in charge of keeping and holding enemy hate while being very survivable. Sometimes they can bring some good damage to the table, too. Almost every party wants a tank, and as such these are in high demand.
Paladin - The classic sword and shield knight. PLD specializes in resisting physical damage, can also heal itself and others. Good at soloing.
Ninja - NIN survives via avoiding damage altogether. Not used as a main tank too often, but makes for a great backup tank and can deal damage more easily than other jobs in this category. They are strong at tanking enemies without too many AOE attacks, as those often bypass NIN's damage avoidance. Unfortunately, the current endgame meta has lots of these AOE abilities, so NIN doesn't see a ton of use as a tank nowadays, but it still can be useful.Good at soloing.
Puppetmaster - PUP doesn't tank directly - instead using its pet puppet as the tank. The puppet has frankly ridiculous amounts of damage resistance, and thus lets your healers focus on other party members more easily. Puppetmasters wield hand-to-hand weapons, but generally let their puppets do most of the fighting. Good at soloing. 
Rune Fencer - RUNs specialize in resisting magic damage and status effects, making it very strong in the current endgame. They can only resist a few elements of magic at a time, so the RUN will need to be familiar with what specific encounters require. With strong enough gear, can also dish out very good damage. Despite the name, they wield Great Swords. 
Notes: Warriors and Monks can also function as tanks in emergencies or specialized situations.
=Support=
The primary role of support jobs is to buff party members or debuff enemies. These jobs are often very strong, and are key members of any party - getting party invites will likely be very easy.
Red Mage - RDMs use strong single-target debuffs and buffs to cripple enemies and embiggen allies. Can also bring strong physical and magic damage to the table, and help out with healing. Very good at soloing.
Bard - BRDs use songs to give allies a wide range of powerful buffs - accuracy, attack, attack speed, MP regeneration, you name it they can buff it. They can also deal respectable melee damage with good enough gear, and have some decent debuffs such as sleeps and dispels. In very high demand.
Corsair - Gamblers by nature, CORs play a quick push-your-luck game to give AOE buffs to their party. Bad luck can cause a Bust though, which removes the buff from the party (and debuffs you). They can also deal very good physical damage. In very high demand.
Dancer - DNC are primarily light DDs, but they can be useful as a backup support as well. They can speed up allied melees and provide unique debuffs that stack with other supports, such as defense down and evasion down. They can also provide backup healing and debuff removal. Good at soloing.
Geomancer - GEOs use AOE bubbles to give powerful buffs and debuffs, localized to specific areas of the battlefield. Their debuffs can't be resisted, and are very powerful - although the GEO will need to keep a Luopan pet alive in order to maintain these buffs/debuffs. In very high demand due to its strong support effects.
=Healer=
It's the healers' job to keep everyone in the party alive. They are mandatory in almost every party, and thus are in very high demand.
White Mage - The classic healer, WHM specialize in keeping the party alive and removing debuffs from them. It's rare to find a party that doesn't have at least one white mage. 
Scholar - While not as specialized as WHMs, SCHs can provide solid healing as well - and then can switch to offensive spells during downtimes when healing isn't needed.
Notes: Red Mages can also function as solid healers. However, they lack debuff removal and AOE healing, and thus are generally more suited to backup healers.
=Party Composition=
When not solo, you'll usually be in a party of 6 players (counting you), or an alliance of 18 players (three 6-player parties). The jobs that make up these parties and alliances vary depending on the content being tackled, but there are some rules of thumb for making a balanced party/alliance.
A party will tend to have 1 Healer, 1 Tank, 2 DDs, and 2 Supports. The DDs can be heavy, light, and/or magic. Since some jobs can work double-duty in roles, such as Corsair providing both support and DD, there's quite a lot of flexibility in this setup. 
An alliance will tend to have a DD party, a Tank/Healer party, and a Mage party. Each of these three parties will usually have a dedicated healer and support, providing buffs unique to that party (attack speed buffs for DDs, defense buffs for the tanks, etc). The mage party is usually there for crowd control, offering AOE damage or debuffing - most content that requires an alliance will pit you against large groups of enemies, and thus a party dedicated to handling these groups is necessary. Due to the large number of players, alliance composition is very flexible.