In the comic book, Spider-man picks up a an alien life form that replaces his suit and web-shooters (in the comic, Spidie's webs don't come from his body). It turns out that the alien has harmful side affects, so Spider-man ditches it.
The alien, pissed at Spider-man for being rejected, ends up meeting Eddie Brock, a guy that hates Peter Parker because he thinks that Peter made him look bad.
Together, as Venom, they terrorize Spider-man/Peter Parker. Venom is bigger, stronger, just as fast, meaner, doesn't set off Spider-man's spider-sense, knows Spider-man's real identity, and is insane. The only thing that Spider-man has going for him is that he's smarter. I should point out that Venom isn't stupid, it's just that Spider-man is just a bit smarter.
He was the perfect villian for Spider-man.