In Greek Mythology, Cerberus was a three headed hound, a rather large three headed hound, that guarded the gates of the Underworld.
The Underworld was namely Hades. The three headed dog, is a representation of what the Greek Gods feared the most, wild dogs, and therefor were made guards. A bit like bouncers I suppose...
Hades was home to the dead, so if you were living, you was not aloud in.
Party poopers!
But it wasn't all a party for the dead because once they were in, they were not allowed out. Nope they would have to stay there for all eternity.
Another thing the Gods were frightened off was Typhon, who was a god and a fire breathing dragon...oh and papa to Cerberus.
They were a fearsome pair! But let's not forget Cerberus' mother, Ekhidna. She was a delightful half snake and half woman. I say delightful with sarcasm, as her main hobby was to seduce men and then gobble them up like a short bread biscuit.
It is no wonder that Cerberus' ended up as he was with parents like his!