The Titans were the pre-Olympian gods. According to the Theogony of Hesiod, they were the twelve children of the primordial parents Uranus (Sky) and his mother, Gaia (Earth), with six male Titans: Oceanus, Coeus, Crius, Hyperion, Iapetus, and Cronus, and six female Titans, called the Titanides  Theia, Rhea, Themis, Mnemosyne, Phoebe, and Tethys.

Cronus mated with his older sister Rhea and together they became the parents of the first generation of Olympians: the six siblings Zeus, Hades, Poseidon, Hestia, Demeter, and Hera. Descendants of the Titans are sometimes also called Titans.

The Titans were the former gods, the generation of gods preceding the Olympians. They were overthrown as part of the Greek succession myth, which told how Cronus seized power from his father Uranus, and ruled the cosmos with the Titans as his subordinates, and how Cronus and the Titans were in turn defeated and replaced as the ruling pantheon of gods, by Zeus and the Olympians, in a ten-year war called the Titanomachy. As a result of this war of the gods, Cronus and the vanquished Titans were banished from the upper world, being held imprisoned, under guard in Tartarus, although apparently, some of the Titans were allowed to remain free

The Giants, also called Gigantes , were a race of great strength and aggression, though not necessarily of great size. They were known for the Gigantomachy (Gigantomachia), their battle with the Olympian gods.  According to Hesiod, the Giants were the offspring of Gaia (Earth), born from the blood that fell when Uranus (Sky) was castrated by his Titan son Cronus.