Anatomy

The Soul - An incorporeal, living essence, a collection of past lives and memories.
* Souls are protected by a powerful barrier that only divine magic is capable of bypassing to interact with directly.
Essence - A single lifetime, a part of the soul. 
* An unstable essence may return to Eternia in the form of a 'spirit', a mana imprint sent from the spirit realm where the essence will remain, protected by the guardians of the realm. The imprint is linked directly to the essence and its experiences are shared. Magi capable of exorcising spirits may be employed if a spirit haunts a person or place, as they cannot be killed persay, but can be banished from a location or barred from the living realm for a number of years.
* Unlike the soul, it is possible to change or outright destroy an essence without divine magic, typically through corrupt means, but only an incomplete essence that has not been sealed within the soul's barrier.
Mana - The fuel of the body that provides energy and can also be channeled for the use of magic if the person is capable. The soul provides this directly.
Magical Circuits - The circuitry that mana flows through, with creatures of the same species having near identical magical circuits. This varies somewhat among sub-species (Oscuri and Drakanites, for example). More is being uncovered about the magical circuits with each passing generation, and knowing it intricately is vital for things like alchemy and organism-based runewriting. 
The circuits are linked to the soul (which distributes mana on request). While the soul can provide an almost limitless amount of mana, the circuits and body naturally tire from utilizing it.
Body - The skin, flesh, and bones. A healthy body is important for casting magic effectively.