Aaaaanyway. So, you die. Poof, one human minus heart equals dead. 'Cept not, you're a Heartless now. Maybe just a Heartless, maybe more. You might be 'lucky.' No tellin'. So, you've lost your body and your heart, and now you're just a thing. Now, you are the inky black darkness that ran you through. You're a teeny-tiny little thing made of pure darkness. And you've got an appetite for other people's hearts. Just like how that inky black thing that stole yours had a hunger for your heart to begin with.

That's a Shadow, and that's the Heartless. They're things. Not alive, not dead, just...hungry.