A character in the
Marvel Comics universe. He first appeared in Amazing
Spider-Man #129. He was hired by the
Jackal to kill
Spider-Man, but once he realized Spidey was a good guy, he backed off.
The Punisher's real name is Frank Castle. He was a soldier during the Vietnam War (or Conflict, or whatever you want to call it). While having a picnic in the park one afternoon after his return, his family was gunned down when they accidentally witnessed a gangland execution. Frank escaped, and started a one-man war on crime.
Why such a drastic reaction? Well, for a long time, Castle was portrayed as just really pissed off about his family's death. But the extent of his war quickly exceeded everyone that had anything to do with the death of his family. Plus, titles like Punisher War Journal made it clear that Castle was a good soldier during the Vietnam War. He thrived in the heat of battle, like maybe he enjoyed it. So a slight retelling of his origin was in order.
In the limited series Born, Garth Ennis showed the real beginning of the Castle's war on crime. In Vietnam, he picked up a liking for battle and killing. For him, the war couldn't stop. And a mysterious voice (never revealed, but probably the Devil) promised to give him a war that would never have to end. But in deals with the devil there's always a price. The price that Frank paid was his family. After paying the price, his endless war could start. So the trigger for his battle is the same as it always has been. Only the reasons have changed.
His success in his war is largely due to his determination. He pushes himself harder than he should and takes insane risks. In his mind, if the risk kills him, he gets to be with his family, and if it doesn't, his war gets to continue. He has no special abilities, just a vast knowledge of firearms, fighting techniques, and dirty tricks in general. He kills the criminals he comes into contact with, once even gunning down a litterbug (he was crazier than normal at the time). Most of the Marvel Universe's superheroes want to see him brought to justice because of his extremist ways, so he keeps his distance. He died and was an avenging angel for a (thankfully) very short period of time. Now, he's alive again, and killing more criminals. And he most likely will continue to do so for a while.
It's worth noting that the character of the Punisher was at the very least inspired by (if not just straight ripped off from) Don Pendleton's character Mack Bolan, the Executioner. Mack Bolan was also a Vietnam veteran who took on the Mafia, but his reasons were somewhat different. Mack's family had become entangled with and was killed by the Mafia, and he was drawn into his war on organized crime against his will. Compare this to Frank Castle, whose family was killed by the Mafia casually, unknowingly. He relishes the killing, and is essentially a psychotic individual who happens to kill criminals. He inhabits an ambiguous moral zone that is the darkest of greys. The two characters may have started out similarly (so similarly that Pendleton entertained ideas of a lawsuit), but nowadays, the Punisher stands alone as a separate character.