Heart

The heart is a part vital to the complete existence of any living thing. Just as people have hearts, so do worlds.

Hearts apparently give the soul and body emotion and feelings, as well as a conscience. Nobodies, who lack hearts, cannot feel or show emotions.

All hearts have both light and darkness within them, except the Princesses of Heart, who lack darkness and have only light in their hearts.

If an individual, be they good or evil, gives into the darkness of their heart, they will become a Heartless, a being of pure darkness who seek out other hearts. If a person with a strong heart becomes a Heartless, then the body and soul left behind may act with a will of their own. This being is called a Nobody. If the person was particularly strong, that Nobody may even retain its human form and memories, but will still lack true emotions which come from the heart. These specific nobodies are part of Organization XIII.

Memories are also tied with the heart. If a person's memory collapses, such as the Riku Replica, their heart will collapse as well, rendering their body into a lifeless doll. It is therefore safe to assume that, since the members of Organization XIII can remember certain things (such as how to break a heart), then they must have either shadow hearts, which contain memory, or fractions of their old hearts.

It should be noted that the 'heart' is one of the three things that make up existence of a living being. The other two are the 'body' and the 'soul.'

Connection to the Keyblade
It is said that anyone with a powerful heart may wield a Keyblade, but if one forgets he/she ever weilded a keyblade, it is no longer possible to weild it. It should be noted that Roxas is capable of wielding Keyblades, but this is due to him being the Nobody of Sora and indirectly inheriting his ability to wield.