ABOVE: The Titanomachia (the war between the older Titans and the younger Olympians). The Olympians won the fight and took over.
A long, long, long time ago, there was only the swirling void of Chaos. Then, gradually, a few gods emerged: Gaia (earth), Uranus (the sky), Nyx (night), Tartarus (the underworld... it wasn't called Hades yet because Hades didn't even exist), and Erebus (the underworld's perpetual darkness). Nyx and Erebus had kids, pretty much the short-lived forerunners of Pandora's box (Murder, Death, Doom, Deceit, Misery, etc.). After a while, Gaia and Uranus had kids too - 13 of them, the first being the sea god Pontus. The last 12 were called the Titans. Right before they were born, Uranus heard a prophecy that one of his sons would overthrow him, so as each was born, he shoved him/her back into Gaia. Obviously, she was displeased. She gave the youngest, Cronos, a sickle and told him to attack Uranus. His opportunity came when Uranus came to mate with Gaia. Little Cronos, who was still in utero, saw his dad coming, reached out with the sickle... and cut off the part closest at hand, Uranus's privates!!!! These eventually became Aphrodite, and the spilt blood became such monsters as the Furies, Giants, and Harpies. Gaia re-gave birth to the Titans later.
Here are the Titans: Cronos (Titan of time), Rhea (Titaness of the earth), Oceanus and Tethys (Titans of the sea), Coeus (the North Star), Phoebe (Titaness of the Moon), Hyperion (Titan of the Sun and father of Helios), Theia (Titaness of enlightenment), Iapetus (Titan of mortality and father of four sons), Mnemosyne (Titaness of memory and mother of the Muses), Krios (Titan of the stars and father of three sons), and Themis (Titaness of justice and mother of many children by Zeus).
Two children of the Titans, Prometheus and Epimetheus (Forethought and Afterthought), created animals and people. Prometheus did the creation work, and Epimetheus gave each creation gifts. Humans were last in line, so Epimetheus ran out of gifts just before Prometheus created humans. Prometheus wondered what to give them, and, being a very good thief, he decided to steal fire from the gods. Prometheus was successful, but he was caught by Zeus later on. Prometheus was sentenced to be chained to Mount Caucasus in Turkey for 30 years, and an eagle would peck out his liver each day (it regrew overnight). Finally, Hercules rescued him in between two of his 12 Labors.
Cronos had a later encounter that was similar to his dad's downfall. After hearing a prophecy that one of his kids would overthrow him, he decided to swallow each baby once he/she was born. However, his wife Rhea wised up and substituted a rock for the sixth child, Zeus, whom she hid away in a cave. When he was old enough, he and his grandma Gaia made a potion that made Cronos vomit all his kids! Zeus then gained power over the world.