The Titans of Disney's Hercules appeared briefly in the series, but played a much larger role as the supporting antagonists in the film. They were used by Hades to stage an unsuccessful uprising against the gods of Olympus. While being far more than just powerful monsters, the Titans were one of the few beings depicted with the power to challenge and even temporarily defeat a god of Zeus' level when working together.( However, they were powerless once Zeus gained his lightning bolts.)
It was revealed in the series that the Titans are all the offspring of the primordial goddess, Gaia, who shared her childrens' malicious nature but was thankfully much less destructive and spent most of her time sleeping under the earth (which she embodied).
In the beginning of the film, the Titans are introduced via the Muses, who sing a song about the dark days of the primordial earth - where the Titans ruled over a hapless population as cruel and chaotic gods, causing all manner of misery to befall mankind until a young Zeus rose against them and cast them down using his legendary lightning bolts.
The Titans (totaling five) were then tossed down into a seemingly bottomless pit (probably representing Tartarus) beneath the ocean and left for all eternity, that was until Hades came up with a plan to unleash the Titans and use their ancient power to exact his own revenge against the gods of Olympus. When the planets aligned in the proper manner after 18 years as precisely foretold by the 3 Fates, Hades set them free: he sent four of them - Hydros, Stratos, Lythos and Pyros - to Mt. Olympus, while the fifth, a cyclops, was given a "special job" - to keep Hercules busy.
The other four Titans easily took over Olympus, with Lythos battering down the front gates and Stratos sucking all the gods up and capturing them. Hydros and Pyros used their combined ice and lava powers to imprison Zeus and immobilize him. Back down on Earth, Hercules managed to defeat the Cyclops by first blinding him and then tying his ankles together, causing him to fall from a high cliff. It is unknown if this actually killed him. In so doing, he accidentally caused the death of Megara; as the clumsy cyclops fell, a column toppled towards Hercules, and Meg pushed him out of them, and was crushed to death instead.
As a side effect of Megara being injured, Hercules was able to regain his strength because of a deal made Hades that Meg herself would be safe from harm's way. He rode Pegasus to Mt. Olympus where he engaged and defeated the other four Titans. They attempted to run away (except for Stratos, who intends to stand his ground), but Herc grabs Stratos and swung him around to suck up his three remaining brothers. He continued swinging Stratos around before finally releasing him, sending him, Lythos, Hydros and Pyros into space, where they all explode to their deaths.