In Greek Mythology, the gods that ruled the Earth and heavens before the age of man. Eventually overthrown by the more common Greek gods, the Olympians, led by Zeus.
The Titans represented the blind forces of nature worshipped by primitive man before the gods of the great civilisations were conceived. The Olympians had to chain them in the deepest level of the underworld to secure the world for civilisation.
Not all Titans were malign however, one Prometheus rebelled against the gods and stole fire from them, giving it to mankind as a gift he initiated civilisation. For this he was punished by Zeus by being chained to a rock while a vulture pecked at his liver. He remained there till rescued by Hercules.
In general the Titans were seen as evil rebels, and this led them to be cast as the primary forces of evil in the Orphic Mysteries. They rebeled against the Apollonian harmony of a primal 'Eden', brought conflict into Nature and dismembered the supreme spiritual being Dionysus.
On being blasted to bits by Zeus the remains of the Titans along with divine fragments of Dionysus were formed into mankind.