Venom is an alien symbiote that requires a host for survival and in exchange gives the host enhanced powers. It originally bonded with Spider-Man but after it's evil motives influenced Peter Parker he rejected it. The symbiote has merged with many hosts the but most well known is Eddie Brock.
From the time bonded with Spider-Man it absorbed his abilities that can be emulated by it's host. Venom is undetectable to Spider-Man's spider-sense ability but is vulnerable to loud noises. Venom is one of Spider-Man's greatest enemies and has a great hatred of it's past host.