After freeing his brothers and sisters from the stomach of their father, Cronus, Zeus gathers the younger gods at Mount Olympus and wages a war. In the end Zeus and his brothers defeat Cronus and his fellow Titans. They banish all Titans to Tartaros.
The Less Short Story
In order to really grasp the events surrounding the war between the Titans and the Olympians, it helps to understand how the whole thing got started.
So, at the beginning of time, there were only three beings: Chaos, Gaia, and Eros.
Gaia is kind of like the goddess of the earth, and she is also the earth itself. Eros is the god of love and may or may not be the same Eros who is born later to Aphrodite. Chaos is the god of… well… Chaos. Got all that?
In time, Chaos gives birth to Erebus and Nyx, darkness and night, and these two give birth to a lot of people that we don't really care about right now. Then Gaia gives birth to Uranus (Ouranos), who represents either Heaven or the sky, depending on which version of the story you're reading. No version of the story explains how all of these births happened without anyone doing the deed, but maybe that's beside the point.
Speaking of which, Gaia and Uranus proceed to get it on. A lot. Don't think about the fact that they're mother and son – it'll just give you nightmares. Anyway, they get busy and a short time later Gaia gives birth to the Titans: Oceanus, Coeus, Crius, Hyperion, Iapetus, Theia, Rhea, Mnemosyne, Phoebe, Tethys, and finally Cronus. (There will be a quiz later.)
Now, you're probably thinking to yourself, "But what about Atlas and Prometheus and those other Titans I've heard about?" Good question. The answer is that Atlas and most of those other famous Titans are actually the sons and daughters of the original Titans. Atlas, for example, is the son of Iapetus and Clymene. True story.
In addition to the Titans, Gaia and Uranus also give birth to three Cyclopes: Brontes, Stereopes, and Arges.
Then (yes, there's more) they give birth to three Giants: Cottus, Briareos, and Gyes.
These Giants were said to have fifty heads and a hundred arms each, and came to be known as the Hecatonchires, which means something like "with one hundred hands." (Makes sense.)
Papa Uranus fears and despises these six powerful creatures, so as they are born, he locks them away inside the earth, laughing at their imprisonment. To be fair, if we gave birth to giants with fifty heads and a hundred arms, we might lock them away too.
Gaia can't bear to see her children mistreated, so she calls upon her first born(s), the Titans, and demands that they take revenge on their father for his cruelty.
This is where we see the first major difference between versions of this story. In Hesiod's version, Cronus is the only Titan brave enough to answer Gaia's summons and as a result he carries out the punishment on his own.
In later versions of the story, Coeus, Crius, Hyperion, and Iapetus work together to hold Uranus down for Cronus. Team effort, you know.
Whichever version of the story you follow, the ending is the same: using a flint dagger, Cronus attacks his father and cuts off his, um, manhood. And then he throws it into the ocean. Yikes.
Consequently, when Uranus' you-know-what is swallowed up by the ocean, it gives birth to Aphrodite. Check out the deets on that lady for more details.
With Uranus unmanned, Cronus ascends as leader of the Titans and is married to his sister Rhea. Yep, incest runs in the family. So Cronus and Rhea get it on. A lot. (Sensing a trend?) Just wait.
In time Rhea gives birth to the first Olympians: Hestia, Demeter, Hera, Hades, Poseidon, and (dun, dun dun!) Zeus.
This set of births isn't all sunshine and daises though. Shortly after the castration, Uranus and Gaia reveal to Cronus a prophecy, claiming that Cronus will eventually be overthrown by one of his own children. Uh oh.
Determined to thwart the prophecy (i.e. to prove it wrong), Cronus devours each of his children the moment after they're born. Yum. So Hestia, Demeter, Hera, Hades, and Poseidon all get eaten whole, which not only stinks but completely defies the laws of physics.
Zeus presumably would have suffered the same fate as his siblings except that just before his birth, Rhea begs her mother to step in.
Gaia takes pity on her daughter and rescues the infant Zeus at the moment of his birth, hiding him away in a cave. Gaia then gives Rhea a stone about the size of a child, and Rhea gives the stone to Cronus who swallows it whole. He's been duped. Who knew that babies actually taste just like rocks?
So Zeus grows up in safety somewhere in Crete, and when he finally comes of age he returns to confront his father. Apparently kicking the crap out of your dad is some kind of rite of passage.
Zeus starts the battle by freeing his brothers and sisters from Cronus' stomach, where they must have been hanging out of for the last however many years – probably giving Cronus some major indigestion.
No one can quite agree on how exactly the Olympians were released from Cronus' belly, and so there are a bunch of versions of this story.
Hesiod wrote that Gaia somehow tricked Cronus into spitting up his children. Later versions suggest that Zeus received a potion from the goddess of wisdom, Metis, and that the potion is what made Cronus hurl up the gods. Either way, with his brothers and sisters at his side, Zeus declares war on Cronus and his fellow Titans. And here we are: the war of the Titans.
Most accounts suggest that the war lasted ten solid years without either side gaining an advantage.
Finally, after a decade of fighting, Gaia reveals to the Olympians another prophecy: if Zeus releases the Cyclopes and the Hecatonchires from their prison in the earth, then the Olympians will win the war. Prophecies of winning are clearly the best kind, so Zeus wastes no time following Gaia's advice.
Once freed from their ancient prison, the Cyclopes make a gift to Zeus, giving him the thunder and lightning bolts that have become his trademark. Meanwhile, the Hecatonchires pledge to join the Olympians in combat, hurling massive boulders at the Titans with all one hundred of their arms.
Hesiod makes a big deal about the thunder and lighting, but personally we think it was the hundreds and hundreds of boulders flying through the air that really tipped the scales. Either way, the war between the Titans and the Olympians comes to a very quick and very decisive end.
Cronus and the Titans are confined to Tartaros, a stinking pit that sits at the very edge of creation.
Zeus is made king of the now reigning Olympians; just in time to marry his sister, Hera, and start pumping out kids. (We told you the incest wasn't over.)
The Hecatonchires are set to guard the gates of Tartaros, and Zeus divides the rest the universe between his brothers and sisters, giving each an office to fulfill.