In Homer's The Odyssey, the Scylla was a vicious sea monster that patrolled one side of the Straits of Messina (sometimes known as the Roving Rocks) and attempted to eat sailors on ships who pass through. She had six ugly heads with three rows of sharp teeth on the end of long, flexible necks which she used to snatch up sailors. Four snapping dog heads encircled her waist, and she had twelve feet.
Ships passing through the Straight of Messina/Roving Rocks between Sicily and Italy had a choice; sail closer to Scylla, or closer to her partner, Charybdis. If they chose Scylla, then she would snatch up six sailors, one for each head, from the deck of the ship. (Any attempt to cheat her would result in her picking up the whole ship and bringing it to her lair.) Charybdis, the whirlpool, would swallow thousands of gallons of seawater at random times and then belch the water back out an unspecified time later. If a ship was caught here at the wrong time, it would be swallowed whole.
Circe advised Odysseus to sail closer to Scylla. He did so and lost six men, but was able to continue his journey home.
Scylla had once been a beautiful sea nymph, and different stories tell of her transformation. In some stories, she boasted her beauty and compared it to the Gods - not generally a good idea - and in another Circe tricked her lover into poisoning her out of jealousy. Either way, the result was not good for either Scylla or Ancient Greek sailors.