A character in the
Marvel Comics universe. He first appeared in Amazing
Spider-Man #300. His real name is Eddie Brock, who first appeared in Web of
Spider-Man #18, before he became
Venom.
Venom is born of a melding between an alien symbiote and Eddie Brock. The alien symbiote was something Spider-Man picked up during the Secret Wars, thinking it was a costume. When he found out it was alive and attempting to bond with him, he rejected it, using its weakness to sonics (in the form of a ringing church bell) to drive it from his body.
Eddie Brock was a reporter who did a big story about the secret identity of the Sin-Eater, a vigilante. When Spider-Man unmasked him, Eddie's story turned out to be wrong. This led to his credibility being ruined. He was disgraced, a fate which he blamed on Spider-Man. Contemplating suicide in the same church that Spider-Man got rid of the symbiote in, he came upon the alien being. Sharing an intense hatred for the webslinger, they bonded, and became Venom.
This bonding gave Eddie Brock knowledge of Spidey's secret identity. It also gave him super strength and such, plus he doesn't set off Spidey's spider sense. He has used these abilities to torment Spidey mercilessly. In a battle with Styx and Stone, Eddie's symbiote appeared to die. It broke him out of jail months later, leaving behind a spawn which bonded with Cletus Kasady to create Carnage. Venom hates Carnage, who is completely insane. He became a vigilante (meaning almost a good guy) for a time, but that ended after another extended seperation from his symbiote. He has since re-joined with the symbiote. The death of his ex-wife may drive him back to villainy, since he blames it on Spider-Man. He's got quite the persecution complex, I know. His symbiote is M.I.A. once again, but because we didn't see it die, it's probably still around.