In the beginning there was only Chaos, but from Chaos the first three primordial beings were born: Tartarus, Gaea, and Eros
Then from Gaea came Uranus, the god of the sky. The two primordial beings were then married.
From Gaea and Uranus came their children: the three cyclops and the three hundred-headed/hundred handed men ( who Uranus cast into the underworld ). Then the the twelve titans were born after them.
Gaea, grief-stricken that her children had been cast into the underworld, begged her twelve titan children to help their siblings. Only the youngest of them, Cronus, stood up to their father.
Cronus, after defeating their father, married his wife Rhea. Though after hearing the prophecy predicted by his mother, when presented his first born child. He ate the child, and the next four that came.
Rhea then sought out help from Gaea to hide their sixth child from Cronus. After hiding the child, who came to be Zeus, he defeated his father and freed his five siblings. They came to become the first Olympians.