Now that Spider-Man 3 is out, the black costume and Venom are all the rage. A lot of people have been asking what the deal is with the symbiote, so I'm going to set the record straight. The movie, as most superhero movies do, over simplifies the symbiote. The symbiote amplifies anger and feeds off of it. Eddie Brock is angry, Peter Parker makes the symbiote angry, Venom is born. Simple.
In the comics, it's much deeper. Symbiotes don't feed on rage, they feed on adrenaline, which can be created in many different ways. The symbiotes as a species take over a planet and force the inhabitants to do death defying (and sometimes not defying) activities in order to feed on the adrenaline. After a while, the feeding kills the host. A symbiote simply called "the Other" preferred to find a single host and bond with it permanently, and was imprisoned for insanity. Long story short, his prison was scooped up by the Beyonder during the Secret Wars and it found it's way to Spider-Man.
After determining that the symbiote wanted to bond forever, Spider-Man forced it off and it fled to Brock. Since the symbiotes tend to take on traits of their hosts, the Venom symbiote became an anti-hero (a lethal protector, if you will) because it has Brock's hatred of Spider-Man and Spider-Man's sense of justice and honor. In reality, the Venom symbiote was actually angry with Spider-Man because it felt Spider-Man was the symbiote equivalent of it's true love. Though Brock's adrenaline-producing cancer allowed it to stay with Brock for a long time, it really longed to be with Parker.
As Venom, Brock periodically left the symbiote after having doubts, but they always ended up back together. However, when Brock learned he had cancer, he left the symbiote, possibly forever. It eventually found it's way to the Scorpion, whose body it essentially took over. At this point, the symbiote seems to be truly bitter that no single person would choose to bond with it permanently.
I hope that clears that up. Not bad for not posting in over 5 months.