It all started with Chaos. From within or without of Chaos came into existence Gaea (female, Earth), Tartarus (male, 'beneath') and Eros (male, sexual attraction). These are the primordial (self-made) beings.
From Chaos also sprang Erebus (darkness, gloom) and Nyx (night). These two figures were actually born of Chaos and are not primordial.
From Gaea came Uranus (heaven, sky, male), Mountains and Pontus (sea). From the bonding of Uranus and Gaea came the twelve Titants, the Cyclopes and the Hecatonchires (hundred-handed).
These beings lived an incredibly long and uncomfortable existence within Gaea's womb. Uranus (with the spunk only a god posseses) refused to end intercourse and give his children the opportunity to be born. His logic was that if his children were born one of them may be powerful enough to overthrow his position as the masculine ruler of the universe. But, of course, the pieces don't always fall where you want them to. Cronus castrated his father with a sickle.
Cronus took his father's position as ruler of the universe, lord of the sky, and mate of Gaea. Their bonding produced the first generation of Olympian gods: Hestia, Hades, Demeter, Posiedon, Hera, and Zeus. Cronus, fearing one of his children would overtake him like he overtook his father, ate all of his children. He reasoned that he would have better control of them within his own body than within Gaea's.
Of course his logic, too, was flawed. The last of his children, Zeus, was switched at birth with a rock. Cronus, nothing more than an brutal ogre at this point, ate the rock thinking it was his son. When Zeus was grown he overtook Cronus and became the permanent and most successful sky god. Zeus marries Hera and they had a son: Ares, the god of war. From his six affairs spring six more children: Athena, Apollo, Artemis, Hermes, Aphrodite and Dionysus.