In the beginning there was only the Goddess Gaia (earth) and the Sky God (Uranus). From the union of the two (where earth meets sky) came many babies (the titans and many different beasts and monsters including the hundred-handed giants and the cyclops). Uranus did not want to see these children and forced them all to live underground so he would never have to see them (not exactly an ideal father).
The Goddess was very sad at the way he treated her children and asked them to rebel. The youngest of the 12 titans agreed and the next time Uranus tried to lay down with his mother he rose up and attacked Cronus (I won't go into the details since they would probably be asterisked out) and drove him away permanently. Thus began the rule of the Titans with Cronos as their leader.
Each of the Titans was named after a fundamental force in the world: forethought, oceans, memory... Things went on for awhile until Cronos began to fear his own child would someday rise up to overthrow him. Every time a child was born he would swipe it up and swallow it whole so it would never be able to challenge his rule.
Zeuss's mother was also saddened by this, and substituted a rock for baby zeuss which was swallowed whole. After Cronos left Zeuss was given to the neriads (rivers) to raise. He eventually grew up and was discovered by his father who agreed to take him in as a lowly servant to prove his loyalty, Zeuss (being clever) poisoned Cronos's drink and made him ill, whereupon he regurgitated all of the Olympian Gods he had swallowed (they were divine and were still very much alive). Thus began the war of the Titans whereupon the Olympians enlisted the help of the cyclopes and the hundred-handed ones and others to overthrow the Titans.
There are many ways to view the story, as a parable, creation story or an oversimplification of life developing on earth. If you want more specifics try the link below or the original creation stories.