"Souls" in the common parlance are magical field patterns that are tied to and represent or are the identities of sentient creatures. Many religions believe that a being's soul, upon the body's death, moves into some sort of alternative reality; as these claims can be confirmed only be necromancy, though, evidence one way or the other is generally not trusted, leaving the matter of afterlives to faith as far as most people are concerned. Souls are, however, involved in a variety of more common forms of magic, both benevolent to the owners of the souls and otherwise.
While souls are associated with sentient entities, with a minority extending the definition to cover nonsentient creatures and a few individuals also including plants, that distinction is one of ideology and/or linguistic convenience; in truth, while a few have claimed otherwise for one reason or another, the present and historical consensus is that there is no qualitative difference between the "soul" field patterns of sentient beings, the field patterns of plants, or indeed the field patterns of small pebbles, not is the magical field between the patterns of one thing and another discontinuous under normal circumstances. There is usually enough difference for an assumption of discrete souls, separate from each other and distinct from the patterns of the inanimate and less complex lifeforms, to be a useful approximation for many applications, though.
Most life is matter-based, with core life processes carried out by a body composed of matter and thoughts existing primarily as processes within a physical brain or equivalent. Matter-based lifeforms generate their souls naturally through the impact their bodies have on the magical field, and, while the coupling once established permits the exchange of information and energy in both directions, allowing some changes in the soul to cause changes in the body, a matter-based entity's soul, if removed from the body, lacks the machinery of life and quickly disintegrates (or, according to many religious individuals, either travels or is copied to an afterlife) unless that machinery is provided for it; the body, by contrast, can regenerate parts of the soul or even, theoretically, completely regrow it (this has been demonstrated a few times under laboratory conditions with simple souls, but in practice it is almost impossible to completely remove a soul without doing so much damage to the body that it is killed anyway; doing so would also be pretty much useless, as it would be easier (though still very difficult) to copy the soul after only scanning the original).
There are some lifeforms, however, that are magic-based or soul-based (the terms refer to the same thing, but usage varies). While all known such entities still cannot survive for long without some material anchor, that anchor might be a dull pebble or even a drift of water vapor; the anchor need have no machinery of life, as that is all contained in the magical pattern itself. Most soul-based entities, however, tend to have at least some life processes in their anchors; with all the life processes in the field pattern, damage to the physical body may be quickly shrugged off (though the ease depends on the particular type of creature), but damage to the soul heals far more slowly than an equivalent matter-based entity would experience under the same circumstances. Adding complexity to the anchor is a larger investment in it but can still be an overall gain. In addition, while soul-based entities often do not require material sustenance (air, water, food, etc.), many of them cannot or cannot entirely subsist on it, requiring some other, more exotic, energy sources.