Venom is a symbiote (an alien that needs hosts to survive) and he is one of Spider-Man's worst enemies. Venom's first host was none other than Spider-Man, who rejected it with the help of sonic blasts. Venom then proceeded to tempt Eddie Brock into becoming its host with the promise of great power. After Eddie got cancer and decided to reform, Venom found a new host in Scorpion and would later bound with Flash Thompson before eventually returning to Eddie.
Venom grants its hosts many superhuman abilities such as superhuman strength and enormous jumps. After its bond with Spider-man was broken, it retained Spidey's ability to shoot webs, but instead of webs it uses a thick gooey substance of which Venom is composed. When Scorpion was the symbiote's host, he developed a tail that could crush anything in his way.
Venom's main weakness is sonic blasts, which confuse it even to the point of leaving its host. This is how Spider-Man got rid of it when he was its host.