Venom was originally a Spider-Man villain created by artist Todd McFarlane and writer David Michelinie and published by Marvel Comics. The character made his first appearance in The Amazing Spider-Man #300 . Venom is a combination of a human host and an alien symbiote.
During the Secret Wars, Spider-Man found a new costume. Little did he know that it was actually a living organism. He brought the costume back with him to earth where Mr. Fantastic studied it and determined it was a symbiote. When the symbiote tried to permanently bond with him, Peter Parker removed it and thought he destroyed it.
Eddie Brock bonded with the symbiote after it left Spider-Man. Retaining all of Spider-Man's powers and memories, Eddie became Venom.
When Eddie Brock learned he had cancer, he sold the symbiote to a mobster who gave it to his son. Angelo Fortunato became Venom, but the symbiote felt he was unworthy and left him to fall to his death.
The symbiote then found former Scorpion, Mac Gargan. Mac became Venom and joined The Thunderbolts. He later became Dark Spider-Man in the Dark Avengers.
When the government dismembered the Dark Avengers they came in possession of the symbiote. Recruiting Flash Thompson, they created Project Rebirth and attempted to make Venom a superhero. Flash is currently the host of Venom.