In the begining there was only Chaos. After thousands of years Gaia (mother earth) was born from Chaos.  Along with Gaia, a few other entities were formed .  Some of these where Eros (desire) and Erebus (night).   Gaia, by herself, gave birth to Uranus (father sky).  Then she and Uranus had the Titans.  Some of the Titans were: Oceanus (the ocean) and Tartarus (the place in the Underworld where people had done evil where punished). They also had the Hecatonchires(100 handed monsters).  Cronos and Rhea (king & queen of the Titans) had the Olympian gods and godesses.  For the complete story see Zeus or Jupiter.