Venom is a symbiote, a sentient alien, with a gooey, almost liquid-like form. As with real world symbiotes, it requires a host, usually human, to bond with for its survival. After bonding, the symbiote endows its enhanced powers upon the host. When the Venom Symbiote bonds with a human, that new dual-life form refers to itself as "Venom".
The Symbiote went on to merge with other hosts, most notably Eddie Brock, its second and most infamous host, with whom it first became Venom and one of Spider-Man's archenemies.
However, Venom also has a bit of good within him, sometimes acting as an anti-heroic vigilante. Venom will also team up with Spiderman when facing Carnage, a piece of Venom that merged with a completely evil serial killer named Cletus Kasady.