The Titans were a race of powerful giant deities (bigger than the gods who would replace them) that ruled during the legendary and long Golden Age. Their role as elder gods being overthrown by a race of younger gods, the Olympians (led by Zeus), effected a mythological paradigm shift that the Greeks borrowed from the Ancient Near East.
There are twelve Titans: six male Titans and six females, known as the Titanesses. The Titans were associated with various primal concepts, some of which are simply extrapolated from their names: ocean and fruitful earth, sun and moon, memory and natural law.
Among the first generation of twelve Titans, the females were Mnemosyne, Tethys, Theia, Phoebe, Rhea, and Themis and the males were Oceanus, Hyperion, Coeus, Cronus, Crius, and Iapetus. The second generation of Titans consisted of Hyperion's children Helios, Selene, and Eos; Coeus' children Lelantos, Leto, and Asteria; Iapetus' sons Atlas, Prometheus, Epimetheus, and Menoetius; Oceanus' daughter Metis; and Crius' sons Astraeus, Pallas, and Perses. These twelve Titans were ruled by the youngest, Cronus who overthrew their father, Ouranos, at the urgings of their mother, Gaia.
The Titans preceded the Twelve Olympians, who, led by Zeus, eventually overthrew them in the Titanomachy. The Titans were imprisoned in Tartarus, the deepest part of the underworld (ruled by Hades), with the few exceptions such as Oceanus, most being those who did not fight against Cronus.
The classical Greek myths of the Titanomachy fall into a class of similar myths throughout Europe and the Near East concerning a war in heaven, where one generation or group of gods largely opposes the dominant one. Sometimes the elders are supplanted, and sometimes the rebels lose and are either cast out of power entirely or incorporated into the pantheon. Other examples might include the wars of the Æsir with the Vanir in Scandinavian mythology, the Babylonian epic Enuma Elish, the Hittite "Kingship in Heaven" narrative, the obscure generational conflict in Ugaritic fragments, Virabhadra's conquest of the early Vedic Gods, and the rebellion of Lucifer in Christianity. The Titanomachy lasted for ten years.