Okay lets see how big of comic geek I can be...
Two Venom Stories (Original and Ultimate)
Original (in a nut shell):
Peter Parker/Eddie Brock both work for the Daily Bugle. Peter a photographer and Brock a sleezy journalist who loses his job because of Spiderman. John Johnna Jameson's son, Astronaut, mission to moon unkniwlingly picks up space alien symbiote. On return to Earth, shuttle crashes, Spidey comes in to save John, Jr. at the same time the alien symbiote attaches to Spidey. Peter later finds the black goo on his suit, and it wraps him up as a black uniform, increasing his abilities quite a bit. Initially Peter likes the suit and increased abilities. The symbiote is evil makes Peter more aggressive. Peter realizes the aliens motives, using sonic blast to rid himself of it. In comes Eddie Brock walking down the street, the alien sense his dislike of Peter Parker, and makes a deal with him, thus Venom is born, and Eddie Brock knows Peter's idenity.
Carnage is the spawn of the original symbiote, the symbiote is asexual, in one of Spideman's successful defeats of Venom the symbiote gives birth to another Carnage. Kletus Kasady a mass serial killer. The new symbiote senses Kasady's inate evil and its a perfect mathc. Caranage is much more deadly and stronger the Venom, and thus turns on him. Venom and Spiderman have to team up to defeat him.
Back to Venom, who has been defeated by Spiderman and left for dead....the symbiote seperate from Brock came back, and even attached to a woman...
The key to remember is that the symbiotes really want to be back with Peter, but as the saying goes, "if i can't have it, no one will"
Ultimate Venom
High Schooler Science Genius Peter Parker, befriends a college Science Geek Eddi Brock, and begin messing with Brock's father's life's work, sometype of symbiotic organism. Which like the original, attaches to Peter. Peter then goes on a crime stopping spree and accidently hurst some people. Peter confesses everything to Eddie (who had already kinda figured it out), telling him about him being Spiderman, because they are close friends. Brock feels betrayed, but seemingly allows Peter to destroy all samples of the symbiote. The rest is self explanatory, (Keep in mind that Peter likes Gwen Stacy, and Brock has attempted to hit on, thats right Mary Jane is just a friend in the Ultimate Series, yes they dated, but only shortly, Peter now dates Kitty Pryde) and I have yet to get up to the Carange story line in the Ultimate series...
Keep in mind its been a while since I read the original Venom story line, so I may be getting some confused with the cartoon (lol)
As much as I want him in the movie, I would prefer it be more accurate with the comic. Being that it was on Spiderman first before it tried to take over him completely and then later to find Eddie Brock (who becomes Venom). But hey there is so much you can squeeze into a movie, so Venom I hope it is afterall. 
:
