A soul is what separates a being from a thing. In the basis, it contains the magical, mental and emotional aptitudes of a living being, but is influenced by upbringing, experience and a whole host of other factors to encapsulate everything about a person (or otherwise sentient) that makes them who they are. It also doubles as a person's storage for energies and contributes to their natural production.
Call of the Soul can summon forth a soul of a living being in the form of a ball of lights and colours that denote the general weal of a person. In its natural state though, a soul resembles his or her body's natural form, though translucent and mostly unable to affect the world around them. On the mortal sphere, a soul will dissipate eventually, unless shielded by magic. A soul can be stored in Soul Crystals for various purposes, but lady Eiraiha has forbidden to do this with living beings as such would cause their body to die.
It must also be said that souls eventually become a part of Heaven or another afterlife destination in general, the basic soul returning to the cycle of life through the Soulforge. It is highly unlikely for two of the same basic souls to exist in the same world, but with the recent addition of Emmy to Praxis, at least one confirmed case is known.