What is the history of Kingdom Hearts?

Answer 1

If you mean the history of how the game series started, then

Kingdom Hearts was created within the year of 2000, the idea created when two excutives (one from Square Enix and one from Disney) met in an elevator. Using the idea as a basis, a team of people came together and created Kingdom Hearts with Tetsuya Nomura as the director and character designer. From then on more titles and mechandise were created based on the series. For more information please visit Google.com.

If you mean the story within the game, then

Birth By Sleep was the first game. It features three teenagers trying to achieve the rank of Keyblade Master, a title reserved for those who pass the Mark of Mastery exam, a test to find out who can really wield the keyblade. A keyblade is a weapon usually shaped like a giant key, that is wielded by those with powerful hearts (this is the only known requirement, though there may be other things that determine who can wield the keyblade). The three characters are Aqua, Terra and Ventus. Ventus is the youngest, and by the time Aqua and Terra take their test, he is unable to take it with them.

The game starts with Aqua and Terra taking their Mark of Mastery, supervised by Master Eraqus, their teacher for wielding the keyblade. He invites Master Xehanort, another master keyblade wielder who once trained alongside Eraqus in their generation. As the test starts, everything is going fine. However Master Xehanort sabotages the test, causing Terra to use darkness to defeat the enemy Xehanort has created. Master Eraqus passes Aqua but not Terra due to his darkness.

Eventually the trio begins to set out to travel to other worlds. Aqua and Terra are sent out to stop the Unversed, creatures who have suddenly appeared across the universe, and find Xehanort, who has mysteriously disappeared. Against Master Eraqus orders, Ventus follows Terra, through urging of Vanitus, the strange apprentice of Xehanort. Eraqus orders Aqua to watch over Terra, and to bring Ventus back home.

The trio travel out and encounter many trials on their journey. Terra is tempted by many villians across the worlds to use his darkness, but eventually the trio all meet up at the Keyblade graveyard, spurred on by Xehanort and Vanitus. Xehanort reveals his plan to take over Kingdom Hearts using the X-blade, a powerful weapon that cuase the worlds to go to war. It is revealed that Vanitus was created from Ventus's heart and is the darkness that the Unversed come from. A battle ensues, ending with Terra almost losing his body to Xehanort, whose old body is on the verge of dying, Ventus losing his heart due to the destruction of Vanitus, and Aqua creating Castle Oblivion to hold Ventus in until she returns to wake him. Ventus's heart eventually finds it way to a young boy named Sora and remains there to heal itself.

Aqua returns to Radient Garden to find Terra-Xehanort, a fusion of Terra and Xehanort. A battle between the two causing Aqua to fall into the Realm of Darkness while trying to save Terra. She succeeds in saving him on the cost of her original keyblade and armor. Terra then remembers himself as Xehanort and is takien care by a man named Ansem the Wise. Aqua wonders the Realm of Darkness until she reaches the Dark Margin.

For more about the story in the game, look up their titles at Kingdom Hearts Wiki.

Chronological order:

(All games start with Kingdom Hearts)

Birth By Sleep

1 (or just Kingdom Hearts)

Chain of Memories (Re: Chain of Memories)

358/2 Days (Technically this starts before Chain of Memories)

2

Re: Coded

3D: Dream, Drop, Distance

Kingdom Hearts 3 has yet to be revealed although there are rumors of its progress and creation.