The ancient Greek people were polytheist, which means that they believed in many gods (poly=many; theo=god). This set of beliefs, their gods and their stories are known today as Greek mythology. There were two groups of gods in Greek Mythology, the Titans, and the Olimpian Gods.
At the beginning the Titans controlled the earth as they were the elder gods who existed from times immemorial, and they were Cronus, Gaea, Uranus, Rhea, Oceanus, Tethys, Hyperion, Mnemosyne, Thea, Prometheus, Atlas, Iampetus, Crius, Phoebe, and Metis. The Titans preceded the Twelve Olympians, who, led by Zeus, eventually overthrew them in the War of the Titans. The Titans were then imprisoned in Tartarus, the deepest part of the underworld.