Cerberus was the hound of Hades, a monstrous three headed dog, with a snake for a tail and innumberable snake heads on his back. He guarded the gate to Hades (the greek underworld) and ensured that the dead could not leave and the living could not enter. He was overcome twice; for his final labour Heracles had to capture him and Hades said that if Heracles could defeat Cerberus with no weapons then he could take him to Eurystheus. And Orpheus used his musical skills to lull Cerberus to sleep.
Cerberus is the offspring of Echidna and Typhon, along with Orthrus (the two-headed hound of Geryon), the Lernean Hydra, the Chimaera (and in some accounts the Nemean Lion); for further details on some of these creatures see also the labours of Heracles.