The Birth: This green-eyed, grinning gargoyle may have his origins steeped in mystery but two things are certain : He is a deadly vilain; Batman's arch nemisis and he first appeared in Batman #1 (Spring 1940). There have been speculations made regarding the birth of the Joker, that he was fleeing the Batman when he accidently tripped into a vat of toxic chemicals that turned his skin chalk white, his hair green and his smile a grinning red leer. Only the Joker knows for certain. The Joker was one of the original Batman villians that was created by Bob Kane.

The Alter Ego: His real name, like his origins, remains a secret. He may have been Jack Napier, a failed comedian who was coerced into petty robbery following the tragic death of his wife. In that case, he may have been the villain previously known as Red Hood.

The Legend: The Joker, the court jester, was the only one who could get away with saying anything. That was in the days of kings and knights. This Joker similarly thinks he can get away with anything. He has pulled off a number of crimes that defy logic, crimes merely to tease the Batman and then a number of crimes for his own profit. With his base of operations as Gotham City, and Batman as it's dark protector, he has got a special vendetta against Batman.
He is the Ace of Knaves, first and foremost of the Dark Knight's rogues gallery. His crimes range from murder with a capital M, extortion, kidnapping, theft, nuclear terrorism and so on in a never ending list. Whether by electrocuting joy buzzers, acid-spewing boutonnieres or spear-firing gag pistols, the Joker celebrates killing in all its forms. He is clearly a hands-full case for the pyscho ward.
And when it comes to hitting too close to home for the Batman, he is the one responsible for the violent attacks on several of the Bat people. It was the Joker who shot and crippled Barbara Gordon; the former Batgirl (See Oracle) and it was the Joker who brutally murdered Jason Todd, the second Robin (See Robin). And it was the Joker who attacked Lois Lane of the Daily Planet (See Lois Lane), making Superman (See Superman) choose between killing the Joker and saving Lois or standing upto his vow of never taking another life - an action which was the final straw that broke Lois' and Clark's engangement. When thing's got too hot for him, he also sneaked into Boston, interfered with the gang wars and attacked Diana. (See Wonder Woman)
The Joker's signature weapon is his Joker Venom, a fast-acting poison that leaves its victims with a grinning rictus in death, not unlike his own macabre smile. Apart from this he may use anything he can lay his hands on.