The Birth: Back in the First Secret War, when Spidey had really trashed his costume, he was directed by a couple of other heroes to a machine that could supposedly manufacture fantastic clothes. When Spider-Man arrived at what he thought was the clothes machine, it produced a small black ball that initially set off his Spider Sense. The ball then "wrapped" itself around our hero, forming the black Spider costume. Spidey wore this costume for a few months, enjoying it's pleasures of mimicking civilian clothes. It could also produce it's own webbing, relieving Parker of the boring task of developing his Spider fluid.
..... Parker became wary when he realized that his costume was actually an alien symbiote which wanted to bond with him permanently. Mr. Fantastic separated Parker from the symbiote with his sonic blaster and kept it for further study. But naturally, it escaped and found it's way back on Spidey's body. Knowing it was affected by sound waves, Spidey ditched the costume using the old bell tower of the church. The plan worked but the costume escaped and hid itself in the church.
..... Venom's birth begins at this point. The star investigative reporter of the Daily Globe; Eddie Brock, did a series of interviews with the vigilante known as the Sin-Eater. Under pressure from the authorities, Brock revealed the Sin-Eater's identity. But Brock was sloppy and didn't bother to confirm his source's story. As it turned out the guy was just a whako who thought himself to be the Sin-Eater. Spider-Man tracked down the real Sin-Eater and exposed the fraud. This cost Brock his job. Forced to write for sleezy mags since no reputable firm was willing to hire him, Brock went over the bend and began to think that Spider-Man was the reason of his disgrace. Brock was busy contemplating suicide at the church when the alien symbiote, weakened after being rejected by Peter Parker, bonded with Eddie Brock transforming him into Venom.
..... The symbiote was first seen in Secret Wars #8. Eddie Brock had his first cameo in Web of Spider-Man #18 while his conversion to Venom took place in Amazing Spider-Man #300.

The Alter Ego: Eddie Brock was involved in a strained relationship with his parents. He was always seeking approval from his stonehearted father - a thing that he never seemed to accomplish, which is why it became a sort of obsession for him. Because of the Sin-Eater incident, Eddie Brock's life fell apart. He lost his job and was disowned by his parents. No reputable firm was willing to hire a man of his doubtful reputation so he was forced into writing sleeze. Eventually he lost his hold on reality and saw Spider-Man as the man behind all his misfortunes. Only his wife; Ann Weying who we first saw in Amazing Spider-Man #375 seemed to tolerate him during that time, but now she has divorced him and headed for a new start in her life. There were two occasions when Ann bonded with the alien symbiote and was refered to as She-Venom by the NYPD; the first being Venom: Sinner Takes All #2.
In recent times Venom seems to have sobered down, though the symbiote seems to make him more violent. He sees him as the "protector" of the innocents and feels that innocence should be valued and not lost. He also believes that anyone he percieves as evil should die; making him a vigilante of he same league as Punisher.

The Legend: When Eddie Brock bonded with the symbiote, the symbiote sensed his hatred for it's former host and all of Spidey's secrets became known to Brock. Venom, in search for Parker, found Mary Jane and hassled her up. He then went out and confronted Spider-Man and in true comic book style, related his hatred for Spidey and also told him of his origin. But as Brock was permanently bonded to the symbiote, Spidey could not separate Brock from the symbiote with the sonic blaster without endangering Brock's life. Venom then attacked Spidey but was defeated when his organic webbing supply was exhausted. He was remanded to the custody of the Fantastic Four.
While his prime enemy has always been Spider-Man, Venom also detests his "offspring"; Carnage since he stands for everything Venom opposes. When Spidey failed to capture Carnage, he sought the help of the Venom; who was only too eager to help out.
Venom's dual identity is publically known and he is wanted by the cops for a number of crimes.
While Eddie Brock does not any super powers, the alien symbiote which doubles as his costume increases his strength proportionately - which means as Eddie makes himself stronger, his normal senses and strength will be augmented higher (super human levels) as Venom.
Like Spider-Man, Venom can make webbing and swing lines using his costume. The costume also mimics clothing and other matter, and can camouflage Venom rendering him invisible. Because of it's alien origin, the symbiote dulls Spidey's spider sense. However, the symbiote is quite sensitive to sonic and thermal attacks.
He now regards Spider-Man as a necessary evil since he helps protect the innocents as well. And Venom is also seen as a hero and protector of a homeless community of "under people" in San Fransisco.

Amazing Spider-Man #300	Bride Of Venom #2