When a person with a strong heart becomes a victim of a Heartless attack their 'outer shell' left behind becomes an emotionless creature called, Nobodies. Most become unrecognizable huminiod creatures, the strong willed people retain their human look, likely to join Organization XIII.
Nobodies are miserable with no emotions, ( though the Organization pretend to have them) so they want to regain their ability to feel. The only thing in existence that can allow that is Kingdom Heart, a collection of hearts from Heartless defeated by the Keyblade.
A group of Nobodies consisting of thirteen warriors who intend to become thirteen Xehanorts. Each one is possession of a special skill (ie controlling water, flame manipulation). After Sora learned about them he helped eliminate all of them. The group was revived when Master Xehanort returned after Ansem and Xemnas' deaths.