Okay...when the island first got attacked, Riku disappeared or whatever, but Kairi lost her heart. Sora didn't know it at the time, but her heart actually disappeared inside his own. So he went on his way fighting and saving the worlds with Donald and Goofy, but once he learned that Kairi's heart was locked inside him, he...basically stabbed himself to take his own heart, to free hers. When a person loses their heart to the darkness, they become a Heartless, and that's what happened with Sora. But Kairi brought him back to the light and he became human again.

...the thing is, when a Heartless is made, the body and mind and soul of a person has to go somewhere. In our world, those things create a Nobody. Make sense?