Short answers, pulled from the story.
Bill Finger, Bob Kane, and Jerry Robinson are credited with creating the Joker, though their accounts of the character's conception differ. Robinson claimed the concept originated with a joker playing card sketch he produced in 1940, Finger provided the Conrad Veidt image from the 1928 film The Man Who Laughs as the visual basis for the face, and Kane handled the final artwork. All three had died by 2011 without resolving the dispute.
The Joker first appeared in Batman #1, published on the 25th of April 1940. He appeared in nine of Batman's first 12 issues and was nearly killed off in his second appearance before editorial intervention saved the character.
In Alan Moore and Brian Bolland's 1988 graphic novel The Killing Joke, the Joker is depicted as a failed stand-up comedian who agrees to commit a robbery as the criminal Red Hood to support his pregnant wife. When Batman arrives to stop the heist, the comedian leaps into a chemical vat, which bleaches his skin, turns his hair green and lips red, and drives him insane. The Joker himself admits he does not know if this version is accurate, saying he prefers his past to be "multiple choice".
In The Killing Joke (1988), the Joker shoots Barbara Gordon, the former Batgirl, paralyzing her. He then tortures her father, Commissioner James Gordon, using photographs of the attack to prove his theory that one bad day could break any normal person. Barbara Gordon subsequently became the Oracle, a data-gathering superhero informant.
Heath Ledger won a posthumous Academy Award for Best Supporting Actor for his portrayal of the Joker in 2008's The Dark Knight. It was the first acting Oscar ever awarded for a superhero film. The Dark Knight grossed over one billion dollars and was the highest-grossing film of the year.
Joker venom is a liquid or gaseous toxin that causes its targets to experience uncontrollable laughter; higher doses cause paralysis, coma, or death, leaving victims with a grotesque rictus grin. The Joker has used it since his debut in 1940, and only he knows the formula. He has demonstrated the ability to manufacture it from ordinary household chemicals.