The short version of Venom's origin is that the Original Venom was created when Spider-Man brought back an alien symbiote from space and it bonded with Eddie Brock. The origin story has been altered over the years in differnet comic book series, cartoons, and the Spider-Man 3 movie. Over the years, the symbiote bonded with many other hosts (some in alternate Marvel universes) creating different versions of Venom. There are many sites dedicated to describing the in depth origin story and history of Venom: