The Joker is a fictional comic book supervillain, modeled after the lead character played by Conrad Veidt from the 1928 film, The Man Who Laughs. He is an insane criminal with a clown-like appearance who takes comedic delight in violent crime. He is also the greatest adversary of Batman, created by Bob Kane and Jerry Robinson.
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 in the Tim Burton Batman film in 1989). In a 1950s story an origin was told in which he was originally a criminal who called himself the "Red Hood." In an encounter with Batman, he jumped into a pool of chemicals to escape pursuit, and this permanently dyed his skin white and his hair green, giving him the appearance of a ghastly clown.
This "origin" was greatly expanded upon in the 1988 graphic novel, Batman: 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 agreed 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. Grief-stricken, the engineer wished to withdraw from the plan, but the criminals strong-armed him to keep his commitment to them. At the plant, the criminals made him don a special mask to 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 blundered into security personnel and a violent shootout and chase ensued. The criminals were gunned down and the engineer found himself confronted by Batman, who was investigating the disturbance. In panicked desperation, the engineer escaped by diving into an toxic waste vat and swam through a pipe leading to the outside. Once there, he discovered, to his horror, that the chemicals permanently stained his appearance into that of a clown-like being 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 mental transformation and resulted in the birth of The Joker.
The Joker's trademarks are his razor sharp playing cards and the laughing gas Smilex, a chemical that infects his victims with the same ghastly grin as his own.
In his initial appearances, The Joker was a straightforward murderer, much like a typical Dick Tracy villain with a bizarre appearance modeled after the playing card, but with only comparitively mildly eccentric behaviour. As the Batman comics soon softened their tone, the character's emphasis was soon turned to jokes and comedy themed crimes and the Joker became a cackling maniac. He quickly became the most popular villain and was used almost constantly during the Golden Age of Comic Books. 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.
This changed with the success of the 1960s Batman television series which brought the character to the forefront along with the rest of the classic rogues gallery. During that period, the Joker was a persistent but essentially silly and harmless character.
In the early 1970s, the character was profoundly revised in the Batman comic stories by Dennis O'Neil 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. This take on the character has been the predominant one since.
The most recent major addition to the character was the introduction of Harley Quinn, an insane psychiatrist who fell hopelessly in love with the Joker in Arkham Asylum and now serves as his loyal, if daffy, sidekick costumed in a skintight harlequin suit. (Harley Quinn first appeared in Batman: The Animated Series, where the character was so popular that a version of her was added to the Batman comic books as well. The comic-book version of Harley Quinn, like the comic-book version of the Joker, is more dangerously psychotic and less humourously kooky than the animated-series version.)
In other media, the Joker was played in live action by Cesar Romero in the 1960s Batman TV series. Romero refused to shave his rather sizable mustache for the role (despite the fact that The Joker is clean-shaven). 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.