Depends on the campaign setting. In the desert world I did, I used a variant on the Egyptian idea of the soul being multiple parts - people could lose parts of their soul and those parts sometimes corresponded to either mental function or emotional function. Someone could lose the part of their soul that governed compassion and become a heartless monster. Aother might lose the part that governed rational thought and become insane. Undead were created by taking at least the 'spark of life part' and sometimes others; more sophisticated spells took less and left more 'person' there, which is why the higher ranked undead (vampires, ghouls, etc) had more intelligence and personality than say, zombies.
Animals had some parts but not others; it's why they were not self-aware. Awakening an animal was done by creating that part of the soul and putting it in the animal. Animated objects and such were done the same way. Mages and priests could make entire souls if they needed to and were willing to risk it, so they could create life.
There were no 'real' gods in the setting; the gods were just very, very powerful beings that had once lived much the same lives as the PCs, but had managed to instill an 'extra part' of the soul in themselves, making them immensely powerful and able to work directly with soul material. Worship basically was a gifting of part of a persons soul to the gods (it was a part that regenerated itself normally). That was also the part that demons, devils, crazy cultists, etc wanted since it was the 'magic part' that let people work magic. (Wizards did magic by hoarding that part. Clerics did magic by emptying it totally and being filled with much more power by the being they served).
In the Greatwood game, pretty much everything has a soul. Some have small souls barely even there (most individual rocks) while some have very powerful souls that burn with power (dragons).