Titanomachy

The Titanomachy was the great war between the Olympians and the Titans (In Greek: Titan=Titan and Machy=War). It was the war that ended with the Golden Age and the rule of Kronos. It was a war between the children of Kronos (Zeus, Poseidon, Hades, Hera, Demeter and Hestia) along with some other Titans, like Leto and Prometheus; and the Titans, children of Uranus and Gaia. The gods eventually won and divided the world, being Zeus the ruler of the sky; Poseidon king of the sea and Hades lord of the Underworld.

The Titans were throwned in the pits of Tartarus, when, in the end of The Lightning Thief, Kronos comes back.

In the Books
In the end of The Lightning Thief it is made clear that Kronos and the Titans have come back.

In The Sea of Monsters, it is shown a golden box containing the remains of the titan lord.

In The Titan's Curse, Atlas is shown to have risen, but afterwards goes back to holding the sky.

In The Battle of the Labyrinth, Luke Castellan inherits the spirit of Kronos and almost kills Percy.

In The Last Olympian, a similar war as the Titanomachy happens, showing Hyperion, Oceanus and Krios. In this book, the Titans are finally defeated, and Chiron states that Kronos's spirit will not re-form in a long time.