Very little is known about the true background of the man known as The Joker. The Joker's memory is a little foggy about what has actually happened in his life. "If I want a past I want it multiple choice." So the events listed here are not reliable at all and could have no grain of truth to them what so ever. What you read may have never happened or are perversions of actual events.
He once confessed to a psychitrist that he was a victim of a his parents harsh practical jokes, clearly a ploy to manipulate the young doctor's emotions.There is only one account of The Joker's childhood, in it his first name is not known; he is simply addressed as "Junior" and it is assumed that his name was Jack Napier. Jack and his mother lived in rundown a neighborhood not to far from Gotham. Jack's father was never home, always traveling because of work. He had spent time in a mental institution after a nervous breakdown, Jack and his mother lived in fear of his him, an abusive man who beat Junior's mother and insisted every one be happy and smile all the time. They were always thankful when he was away on business.
Jack would get away from all the troubles at home by going to his secret place, a drained water tower filled with his "toys". Jack's toys were skeletons of hundreds of dead animals he would glue together to make odd animals of his own creation. Birds with skulls of kittens, along with kittens with wings. Comminglings of rats and puppies, squirrels with beaks, and other things with eight legs and three heads, and ribcages melted together like strange Siamese twins. All these things Jack had created out of his imagination, and in that water tower he saw a thing that truly made him smile: Death. Jack didn't kill all of the animals he'd found; he estimated just forty of them he'd killed. Normally he would find a dead animal on the side of the road, take it home and put it under a box in his basement, a box filled with ants and roaches that would eat away at he animal’s flesh so he could get to it's skeleton quicker.
Jack always went alone to his secret place alone, but one day, an annoying boy named Wally had followed him to the water tower. Jack panicked. He didn't know what to do. Thinking quickly he invited Wally up and inside where he showed him his toys hanging from the ceiling of the water tower. Jack then hit Wally in the back of the head with a hammer. He fell and Jack watched him lie on the floor and twitch. Junior beat him over he head several more time until he was dead. Jack looked down at what he had done and smiled, he had some new toys to work with.
It's been proposed that Joker's real name was J. Reipan. No one can know for sure, not even The Joker himself. So in this telling of his "origin" he will be addressed as J.J. was a lab assistant at Ace Chemical plant and was married to a woman named Jeannie, who was pregnant. J.J. had a lot of trouble supporting her, so he quit his job to become a stand up comedian. His first night he was booed of stage after screwing up every punch line. 
Not soon after he was very drunk at a local bar, telling all of his troubles to the bar tender that had served him his drinks, not knowing that two men were listening. They approached J.J., telling him meet them at the same bar the next night, saying they could fix all of his problems. J.J. showed up, willing to do anything to support his wife. They had heard him talking about when he once worked at the chemical plant, and they wanted help stealing some high price chemicals. Turns out they were behind the whole Red Hood operation. The Red Hood was a criminal who was responsible for millions in stolen goods, but in reality, each time a Red Hood crime 
was reported it was always a different person, J.J. was told. They wanted J.J. to be the Red Hood and lead them around the plant. J.J. agreed to do it, thinking some thing good is actually going to happen in his life, but just before they left to go to the plant a police officer informs J.J. that Jeannie had a car wreck, she and J.J.'s unborn son had died. Deviated, J.J. tried to back out of the job after what he had just learned. But the boys insisted he go. They shoved the Red Hood mask on J.J. and they were off. All goes smoothly until the middle of the operation when the alarm goes off. The police arrive, and not shortly after, Batman. Two cops shoot J.J.'s accomplices and J.J. is left to face Batman. Scared, he backs into a vat of chemicals and plunges to the bottom. The Red Hood mask acts as an air pocket, allowing J.J. to swim out a drainpipe. Finally out of the plant, he rips off the mask and gets a good look at his new chalk white skin and green hair. The combination of events drove J.J. completely insane and turned him to what he is now: The Joker!
Only the Joker knows for certain.
The truth is this: The Joker has committed sins that defy logic--extortion, mass murder, nuclear terrorism--crimes often committed merely to taunt the Batman, a foe with whom the Joker has taken a very personal interest. It was the Joker who shot and permanently crippled Barbara Gordon, the former Batgirl. And later, in perhaps the most brutal attacks of all, the Joker murdered Jason Todd, the second Robin, as well as Commissioner Gordon's wife, Sarah Essen-Gordon.
The Dark Knight rests easy only when the Joker is safely locked away in Arkham Asylum, and then knowing all too well that a new body count awaits his next escape.
