Ganondorf is the eternal rival of Link, the Hero of Time. He is a depraved tyrant with the powers of a god. He has appeared in almost every Zelda game as the last boss and most imposing enemy. Ganondorf often manipulates events behind the scenes.
Ganon, otherwise known as Ganondorf, was born to the Gerudo tribe (a race of dark skinned, desert dwelling thieves, who are almost all female, like the Amazons). The Gerudo tribe births one male every 100 years, and that male is destined to be king. Ganondorf started out as a good king, but he couldn't bear the harsh conditions his people had to endure while the Hylians (those native to Hyrule, ex: Zelda, and Link) enjoyed warm summer breezes.
Ganondorf began to research tomes containing mystical power in hopes of changing his fate, but fell into corruption doing so. Ganondorf wanted more and more power. It was never enough for him. Through this way, he ascertained the location of the goddesses' power, the Triforce. Ganondorf sought to steal the Triforce of Power to take control of Hyrule and the world. Ganondorf succeeded in infiltrating the Sacred Realm and stealing the Triforce, but was only able to steal the Triforce of Power (he did not possess the qualities to wield Wisdom and Courage). But with only the Triforce of Power, Ganondorf attained immortality. godlike powers, and his Ganon form.
After gaining the Triforce of Power, Ganondorf set in motion his plan to overthrow the Hylian Royal Family and take Hyrule for himself. It was then that he met his eternal rivals; Link the Hero of Time and Princess Zelda for the first time. The hero was powerless to stop Ganondorf at his young age, and the dark king succeeded in his plots.
Ganondorf's dark reign continued for seven years until the Hero of Time reappeared and defeated him, sealing him in the Sacred Realm and bringing peace to the world.
Ganondorf has since reincarnated several times and has been defeated by Link everytime.
Power from the Triforce
Immortality - Ganondorf has been defeated many times only to resurrect himself. It is pretty clear that he cannot be killed, only banished.
Superhuman Strength - Granted by the Triforce of Power, Ganondorf possesses enhanced strength with an unknown upper limit. In Legend of Zelda: Ocarina of Time, he is buried by rubble, but is able to easily break free of it, sending many pieces of debris flying in multiple directions. Many of these pieces are the size of large houses, so it is likely that Ganondorf's strength falls somewhere between the 40-50 ton range.
Sorcery - Ganondorf was already an extremely adept sorcerer before he attained the Triforce of Power, which increased his magic immensely. This gave him the ability to do things like create hordes of evil minions from dark magic, summon demons/ghosts, give powers to others, shapeshift, create shadow dopplegangers of himself, and open up portals to other dimensions.
Invulnerability - Ganondorf can only be harmed by divine weapons of some sort. It should be noted that, in nearly every case, even divine weapons have only been able to stun him or temporarily cause him pain. Link's main weapon, the Master Sword, is the only known thing which can truly kill Ganon. Normal weaponry/force will not defeat him, regardless of its nature.
Energy Projection - Ganondorf is capable of projecting powerful force-bolts of dark magic, and is also able to project nigh-invulnerable forcefields (he has used this technique more than once to trap and contain others).
Possession - Ganondorf is able to turn into an intangible spirit form and possess others.
Flight - Using his magic, Ganondorf is able to achieve flight (the exact speeds at which he can fly is unknown).
Telepathy - Ganondorf is able to speak to others in their minds without being anywhere near them (he displays this ability to Link in LoZ: Ocarina of Time and to his servant, Zant, in LoZ: Twilight Princess).
Swordplay - Ganondorf is an expert swordsman, able to easily keep up with Link when fighting with him.
Tactics - Ganondorf is a master tactician, able to manipulate emotions, people, and events to achieve his own ends.