Keyblade

The Keyblades are mysterious weapons that partially have a will of their own. It appears that there was once a "Keyblade War" in which Keyblades were common weapons. Many Keyblades can be seen in the secret ending of Kingdom Hearts 2, although none of these have chains, and therefore are not "real" Keyblades.

Acquisition
A Keyblade is said to choose its own master. This may be through love, power, or other unknown virtues.

Powers
A Keyblade can utilize magic abilities, such as Fire or Blizzard magic. A Keyblade can summoned back to its master. Keyblades (or at least Sora's) can gain new powers and abilities through different keychains that can be attached to the blade. Keychains also change the appearance of the Keyblade. It also acts like a homing signal to the Heartless. Another feature is that it can lock or unlock any keyhole, not only Keyholes. It is the only weapon that is capable of truly vanquishing the Heartless and Nobodies; otherwise, the Heartless will continue to exist.

Sora's Keyblade
Sora's Keyblade was given to him when his home world, Destiny Islands was destroyed by the darkness. Sora's Keyblade is the Keyblade of the Realm of Light. Its default key chain is the Kingdom Key. It is commonly referred to within the game as "the Keyblade". Sora's key can seal Keyholes. It is yet to be revealed if any other Keyblades have this power. The Keyblade also refuses to let anyone but Sora use it, and instantly returns to his hand when taken by another. Sora is also able to summon it back to him, as seen during his fight against Roxas. This Keyblade's original chosen one was meant to be Riku, but after Riku gave in to the darkness during the attack on Destiny Islands, it chose Sora instead. The Keyblade did return to Riku for a short time, as he was the intended wielder, but it returned to Sora after Sora showed his heart was stronger than Riku's.

Mickey’s Keyblade
King Mickey’s Keyblade is the Keyblade of the Realm of Darkness, which the King recieved sometime during his journey in that realm. It is identical to Sora's Keyblade, except the colors are switched, the hilt being silver and the blade gold. Mickey used this Keyblade along with Sora's to seal the Door to Darkness, so it theoritically has the same powers as the its Realm of Light counterpart. It is unknown exactly how Mickey obtained the Keyblade, but he had it before meeting Ansem the Wise, meaning that he had had it for a while before then.

Riku's Keyblade
Riku's Keyblade is the Way to the Dawn. It seems to be an evolved Soul Eater, but not much is clear about this Keyblade. Riku's Keyblade is the Keyblade of the Twilight Realm, or the world in-between. How Riku received this Keyblade is unknown, as he had the Soul Eater in the Land of Dragons, but it had changed to its Keyblade form by the time Riku met Sora again in The World That Never Was.

Kairi's Keyblade
Kairi's Keyblade was given to her by Riku in The World That Never Was. Kairi's Keyblade is delicate looking, and sports a flower design. Kairi used this Keyblade to fight (albeit inefficiently) against the Heartless for one battle in The World That Never Was. Curiously, Kairi's Keyblade has a chain, meaning that it is a real Keyblade, not a fake like the Dark Keyblade. The chain appears to be a paopu fruit. The fact that Kairi is able to wield a Keyblade raises many questions that Nomura will most likely address in the future installments of the series. Tetsuya Nomura has said the scene in which Riku her Keyblade hints at something.

Dark Keyblade
The "dark" Keyblade was a nameless Keyblade made out of the six hearts of the Princesses of Heart. It had the power to unlock peoples hearts and was used by Riku when he was possessed by Xehanort's Heartless. Riku first used it to unleash Maleficient's dragon form, then wielded it in combat against Sora. This Keyblade disappeared when Sora stabbed himself with it to free Kairi's heart.

Roxas' Keyblade
Roxas, being Sora's Nobody, also has the ability to use the Keyblade, and utilizes the Oathkeeper and the Oblivion Keyblades, which earned him the name "Key of Destiny" from his fellow Organization members.