At first, it was only Chaos. Out of Chaos, Erebus, Nyx, and Eros were born. From Eros came Aether, and Hemera. Once there was Aether and Hemera, Gaea appeared. Gaea alone gave birth to Uranus, who became Gaea's husband. Together they produced the three Cyclopes, the three Hecatoncheires, and twelve Titans.
Uranus was not a good father, or husband. He hated the Hecatoncheires and imprisoned them in Gaea's womb. This angered Gaea, and she plotted against Uranus by getting the youngest Titan, Cronus, to overthrow him.
Cronus became the next ruler, but became paranoid that one of his children would overthrow him, like his father before him, so he ate them. His wife, Rhea, concealed a baby (Zeus) by switching him with a stone in a swaddling cloth. She was able to pass off the rock as the baby. Cronus was fooled and swallowed it.
Zeus overthrew Cronus with the help of all his imprisoned brothers and sisters. After defeating Cronus and his siblings, they became rulers of the gods.
Zeus exiled the Titans who had fought against him, except for Atlas, who was forced to hold the world on his shoulders for eternity.
Zeus now rules as king of the gods, who reside on Mt. Olympus.