The Joker is a character from the Batman Universe and is one of the most beloved villains of all time. The Joker was dreamt up by Jerry Robinson, Bill Finger, and Bob Kane- his appearance was inspired by the actor Conrad Veidt.
In 1940 The Joker made his first appearance in Batman #1 where he was portrayed as a psychopath. In the 1950's The Joker was transformed into a joking mobster due to regulations from the Comic Code Authority that prohibited too violence/gore/darkness in comics.
In the 1970's The Joker returned to his dark roots; most notably in 1988's The Killing Joke by Allan Moore and Brian Bollard. TKJ is noteworthy because it established The Joker's most widely accepted origin story to date. The story goes that The Joker was a failing comedian who forced into criminal activity to support his pregnant wife. He operated under the name The Red Hood until a job went south and he was pushed, by Batman, into a chemical vat which mutated his skin.
It is important to note that The Joker is supposed to be a psychopath and an unreliable narrator. The Joker confesses in The Killing Joke that he has lied so often about his origin that he doesn't truly remember what story is true- claiming "if I'm going to have a past, I prefer it to be multiple choice!" However, the story of him being a failed stand up comedian is reiterated in Shadow of the Bat #37 and #38.
In Batman #38, author Scott Snyder elaborates (and twists) The Joker's origin story even further. Snyder writes of a radioactive meteor that fell to earth in a prehistoric area. The meteor is what grants immortality to both Vandal Savage and Ras Al Ghul. The new origin story claims that The Joker has been around for longer than Gotham and was made immortal by radiation from the meteor. The radiation only surfaces when The Joker is severely hurt- which explains how he survives all of Batman's beatings/ getting his face peeled off in Death Of The Family.