Short answers, pulled from the story.
Bill Finger, Bob Kane, and Jerry Robinson created the Joker character in 1940. Finger provided the writing and inspiration from the 1928 film The Man Who Laughs, while Robinson produced the initial sketch of a playing card that became the character's visual identity. Kane claimed credit for the final design, though their accounts of who contributed what remain in dispute to this day.
The Comics Code Authority, established in 1954 to curb the perceived violence and horror in comic books, forced a drastic shift in the character's portrayal. Writers and editors transformed the murderous psychopath into a wacky, harmless prankster who committed crimes for the sake of a laugh rather than for profit or malice.
The 1973 story The Joker's Five-Way Revenge marked a pivotal turning point when writer Dennis O'Neil and artist Neal Adams returned the character to his homicidal roots. O'Neil deliberately sought to restore the Joker to the dark, dangerous figure he was in the early 1940s, and Adams modified the character's appearance to make him more visually imposing.
Alan Moore and Brian Bolland's graphic novel The Killing Joke introduced the idea that the Joker was once a failed comedian who became the Red Hood to support his pregnant wife, only to be driven insane by a fall into a chemical vat and the death of his family. This story established the tragic origin that has been cited in countless subsequent adaptations, including the 1989 film Batman.
The introduction of Harley Quinn in the 1990s added a new layer to the Joker's character, transforming him from a solitary figure into a partner in crime with a tragic, abusive relationship. Harley, originally a psychiatrist at Arkham Asylum, fell in love with the Joker and became his sidekick, donning a red-and-black harlequin costume to join him in his schemes.
The 2020 miniseries Batman: Three Jokers revealed that there may be three separate Jokers, each representing a different aspect of the character's personality, further complicating the narrative. This lack of a fixed origin allows the Joker to remain a figure of chaos, constantly reinventing himself and challenging the notion of a single, coherent identity.