The Titanomachia was a great battle fought for eleven years between the Titans and Olympians, the first two sets of gods in ancient Greece. Parallels have been drawn between it and the the war of the rebelling angels in Milton's Paradise Lost.
In the earliest days, there was Chaos. Chaos filled everything; by different accounts, it was a vacuum, or a churning broth of the four elements. Out of Chaos came five children: Erebus, the Darkness; Eros, Love; Gaea, the Earth; Nyx, the Night; and Tartarus, the Underworld.
These beings began to mate with each other, as well as produce children asexually. Many show up in later myths, but the most important in this story is Uranus, the Sky. Uranus came from Gaea, and was her lover and her partner, blanketing her and enveloping her.
Uranus and Gaea had many children, including the Hecatonchires (giants with one hundred arms), the Cyclopes, and the Titans. The Titans were considered truly ancient by the Greeks. The Greeks had no problem with the idea that any being that anybody ever had worshiped had really existed, and had indeed adopted several gods from the pantheons of nearby civilizations, but the Titans were so old that almost none had shrines in Greece, and with several, we're really not sure any more what they held dominion over. Gaea's and Uranus' children:
- Coeus, intelligence
- Cronus, the harvest
- Hyperion, light
- Japetus, time
- Mnemosyne, memory
- Oceanus, the ocean
- Phoebe, the moon
- Tethys, rivers
- Theia, glory
- Themis, law
Many of these Titans had children of their own. Among the more famous of these are Atlas (the moon), Eos (the dawn), Helios (the sun), Metis (wisdom), Prometheus (of fire fame), and Selene (also associated with the moon). These deities went out and did great deeds, slew people, and had a whole lot of sex. Things were normal, as far as ancient pantheons are concerned, for a while.
However, Uranus came to despise his children, and had them all imprisoned within Tartarus, deep inside of Gaea. Gaea then made an adamantine sickle, and charged her children to take it up and overthrow Uranus. Cronus, the youngest (so he had the most to prove), and the harvest deity (so he had the most skill with a sickle), took the sickle. He then ambushed Uranus on his way to Gaea, and while his brothers held Uranus down, Cronus castrated his father and threw the testicles and sickle both into the ocean.
From the blood dripping on the Earth, Gaea conceived the Erinyes, or the Furies, as well as the Gigantes and the Meliae (or tree-nymphs). From the testicles came forth Aphrodite, the goddess of sexual love, although it's not clear where the seashell came from. From the sickle arose the island of Corfu, where the Phaeacians would later live.
After this, Cronus took power, paired with Rhea, and imprisoned many of the other great beings, including the Cyclopes, Gigantes, and Hecantochires. A prophecy had been revealed to Cronus by his parents that he, like Uranus, would be overthrown someday by his child. As a result, he swallowed every child born to Rhea — Demeter, Hades, Hera, Hestia, and Poseidon. A sixth child was born to Rhea, named Zeus, but she hid him away so that he would not be swallowed, giving Cronus instead a large cloth-wrapped stone, which he promptly ate.
When Zeus was full-grown, Metis supplied him with a potion that he forced Cronus to drink, resulting in him vomiting up the stone and five gods. He then murdered the monster Campe, releasing the beings that Cronus had imprisoned, and soon began the Titanomachia, the struggle of the new generation of gods (later to be called the Olympians) and their allies against the Titans.
The war was fought through ten years, and completed in the eleventh. In the end, it was the alliance with Gaea's non-Titanic children, as well as thunderbolts, gifts given to Zeus from the beings he liberated, that turned the war and won it for the Olympians. They then punished the Titans, chaining them and burying them in Tartarus.
Atlas, Epimetheus, Menoetius, and Prometheus were brother Titans; although Atlas and Menoetius stayed with the Titans in the war, their brothers turned to the side of the Olympians and thus were not punished at the war's conclusion. Atlas was made to hold the sphere of the heavens on his shoulders, though Menoetius was condemned to Tartarus. Prometheus later got into trouble for his own deeds, as you may well know.
Thus began the rule of the Olympians. Six other major gods were born, including Athena, who was the result of Zeus swallowing the pregnant Metis, and Hera, who was to become Zeus' wife. It was prophesied that someday, Zeus too would be overthrown by his son, but to the best of my knowledge, this is only played out in a video game (God of War II, to be specific); I have not heard any classical myth describing this event. Later, the Gigantes would wage war on the Olympians in the Gigantomachia, but that is a story for another day.