Kingdom Hearts is a serial of video games developed and published by Square Enix (which used to be SquareSoft) which is an association between Disney and the universe of Final Fantasy, a big serial of RPG also created by Square Enix. Thus, the games use almost all the characters from the cartoons and the movies of Disney (Mickey mouse, Dingo, Donald, Tarzan, Princess Ariel, Heracles...).
On the other hand, you will find Tidus and Wakka (Final Fantasy X), Cloud Strife and Squall Leonhart (Final Fantasy VII)...

Kingdom Hearts is an action-RPG game (so the fights happen in real time) trying to recreate as faithfully as possible the Disney universe and Final Fantasy at a time. The surprise is the fact of having a real and coherent scenario which manages to justify the presence of these completely different worlds. Hence, the scenario of the game is quite original.

Sora (the hero of the game) and his friends Kairi and Riku live on the peaceful island of Destiny. One night, the Sans-Cour invade their place, and Sora lands with a Key-Blade in an unknown world. Then he meets Donald and Dingo, who are looking for their king Mickey.