A Discourse on Souls

The soul is what makes you, you. You are your soul. Your body is just the vessel that your soul is inhabiting at this time. The basis of many transmutation magics is that you alter the vessel, the soul remains constant. When you polymorph into a t-rex, you don't become a t-rex, your soul's vessel has been morphed into that of a t-rex (potentially limiting the thoughts you can process, because of the mental statistic change, but you still are "you").
All creatures have a soul, sapient or no. Whether human, elf, horse, dog, treant, dragon, giant, all sentient creatures have souls.
When your vessel expires, whether it be due to age, wounds, sickness, etc, your soul does not cease to be. This is the reason creatures can be resurrected. The vessel is repaired, and the soul called back to it. That's also why resurrection requires the soul to be willing, as the soul is the being, and can be unwilling to return (perhaps they feel that their time has come, or something more).
A soul without a vessel can become many things. Souls with unfinished business, but no vessel to carry it out may become Spectres, Ghosts, and the like, allowing them to continue influencing the Material Plane. Souls bartered with may end up in the hands of a Devil, who is free to use it for its twisted purposes. Souls of devout believers may be given the angelic form of a Deva or a Solar in the Plane of their deity. Souls that have no definite resting place return to the Mother of Ravens, the Shepherd of Souls. She welcomes all souls to her embrace, regardless of the life they lived. Her flock spans the history of the world, from first life to the present day.
The Mother of Ravens has her place in new life, as well as death. When a new life is created, the soul that will inhabit this new vessel is created from the souls of all who have come before. The personality of the new creature is a mix of the souls that make up this new being, and the parents/creators of it. Sometimes, a new soul will resemble an old one so much so that it may well be a reincarnation of the previous being. The memories of a soul's previous life rarely carry into a new life but may influence the new being's thoughts when it encounters a situation similar to something it has experienced before. These somewhat foreign memories are frequently referred to as déjà vu.
In the beginning, when the gods first created all creatures, each creature carried a new soul, crafted by the gods themselves, and containing a trace of that god's soul. Each creature's soul is made up of aspects of souls from generations before. This amalgamation of souls is not limited by gender, race, or species.