What’s with the dark suit?
This isn’t just any ordinary suit; it's in fact an alien life form called a symbiote. These are organisms that bond with living animals then feed off the host’s adrenaline in order to survive.
Where did it come from?
In the comics, Spider-Man picked it up in Issue #8 of Secret Wars, after his original suit was destroyed. Produced by a strange machine, it oozed over his entire body giving him his edgy new look. In the film, it comes into contact with Spidey after it’s brought back to earth by astronaut John Jameson, son of J. J. Jameson (he almost got married to Mary-Jane in the movie Spider-Man 2).
What does it do?
Initially it has a positive influence, as Spidey in the comics discovered it produced its own web fluid. Eventually though, it bonds permanently with its host, meaning it starts controlling their actions and preventing them from ever changing their boxers again. In the film it also affects Spider-Man’s personality, bringing forward a nastier, more violent side to Peter Parker, which affects his relationship with Mary-Jane.
How do you get rid of it?
In the comics, Dr. Reed Richards (Mr. Fantastic) discovers that it is vulnerable to sound and breaks the bond with a sonic wave. Unfortunately the symbiote defeats Spidey and re-establishes the bond, forcing our hero to a bell tower, where deafening noise makes the separation permanent.
What’s this got to do with Venom?
Well, after the separation from Spider-Man, the symbiote looks for a new host and is drawn to the hatred burning inside Eddie Brock Jr. He has a personal beef with Peter Parker for professional and romantic reasons. The result is the rampaging ball of fury that is Venom.