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The Soul is the metaphysical "glue" or energy source for a being. We could say that it keeps your state of being in one piece and provide a sort of longevity and existence for a person or being. The soul then ties together the Mind and Body. So, we see the trinity: Soul, Mind, and Body. The three things can be separated, but it creates different states of being. For example, a Body can have a Soul, but can be without a Mind, i.e. a mindless undead. An undead, though, does not need either a Soul nor a Mind to function.
In many cases, Souls can be replaced with other forms of energy to accomplish a similar effect, but the soul will always be strongest, since it typically holds the empathic energy of a creature. Necromancy magic or other forms of magic can create a sort of fuel for a body to move on its own. You could think of the "soul" as gasoline to a car, where any other substitute form of energy such as magic/necromancy could be considered a "lower-grade" gasoline.