Gaea is the Earth goddess. She mated with her son Uranus to produce the remaining Titans. Gaea seems to have started as a neolithic earth-mother worshipped before the Indo-European invasion that eventually lead to the Hellinistic civilization.
Uranus is the sky god and first ruler. He is the son of Gaea, who created him without help. He then became the husband of Gaea and together they had many offspring, including twelve of the Titans.
His rule ended when when Cronus, encouraged by Gaea, castrated him. He either died from the wound or withdrew from earth.
Rhea was the wife of Cronus. Cronus made it a practice to swallow their children. To avoid this, Rhea tricked Cronus into swallowing a rock, saving her son Zeus.