Cerberus was the vicious, three-headed dog who guarded the entrance to Hades. He prevented living souls from entering the nether world and the shades of the dead from leaving it. His den was located on the far side of the Styx, at the spot where where Charon landed his ferry. His monsterous appearance is quite understandable considering his parentage. He was the son of Typhon, a fire-breathing giant covered with dragons and serpents, and Echidna, half-woman, half-serpent.
Cerberus appears in several myths, such as Orpheus in the Underworld and the Twelfth Labor of Hercules.
This link to Mythography provides additional information about Cerberus in myth and art.