Joker (character)

A comic book supervillian who is the greatest adversary of Batman created by Bob Kane and Jerry Robinson. He is an insane criminal with a clownlike like appearance who takes comedic delight in violent crime.
The definitive origin and actual name for the character was never established in the comics (although many now assume his real name is Jack Napier as from the Tim Burton Batman film in 1989). However, the most creditable account was told in the graphic novel, The Killing Joke written by Alan Moore. In that story, The Joker was an unnamed engineer who quit his job at a chemical company to become a stand up comedian, only to fail miserably. Desperate to support his pregnant wife, he agrees to guide two criminals into the plant for a robbery. During the plan, the police come and inform him that his wife had just died in a household accident. Griefstricken, the engineer wishes to withdraw from the plan, but the criminals strongarmed him to keep his commitment to him.
At the plant, the criminals make him don a special mask become the infamous Red Hood. Unknown to the engineer, this was simply a way to implicate any accomplice as the mastermind of a crime to divert attention from themselves. Once inside, they almost immediately blunder into security personnel and a violent shootout and chase insues. The criminals are gunned down and the engineer finds himself confronted by Batman who was investigating the disturbance. In panicked desperation, the engineer escaped by diving into an toxic waste vat and swamp out a pipe leading to the outside. Once there, he discovered to his horror that the chemicals permanently stained appearance into a clown like appearance with chalk white skin, ruby red lips and bright green hair. This turn of events compounded by the man's misfortunes on that one day caused his mind to snap and he became The Joker.
Originally in the initial appearances, The Joker was a straightforward murderer much like a typical Dick Tracy villian with a bizarre appearance modeled after the playing card, but with only comparitively somewhat eccentric behaviour. As the Batman comics soon softened their tone, the character's emphasize was soon turned to jokes and comedy themed crimes and the Joke became a cackling maniac. He quickly became the most popular villain and used almost constantly during the Golden Age of Comics. The use of the character lessened somewhat by the late 1950s and disappeared almost entirely when Julius Schwartz took over editorship of the Batman comics in the mid 1960s. 
That changed with the success of the 1960s Batman television series which brought the character to the forefront along with the rest of the classic rogue's gallery. During that period, the Joker was a persistent, but essentially a silly and harmless character.
In the early 1970s, that was profoundly revised in the Batman comic stories by Dennis O'Niel and Neal Adams. Beginning with the story, The Joker's Five Way Revenge, The Joker became a homicidial maniac who casually murdered people, even his own henchmen, on a whim but enjoyed the battle of wits with Batman. That take of the character has been the predominate one to this day.
In other media, the character was played in live action by Cesar Romero in the 1960s TV series. Jack Nicholson played the role in the feature film Batman directed Tim Burton. In animation, Mark Hamill is the most famous actor to play the character's voice in Batman: The Animated Series.