The existence of the soul is heavily debated in the first place, and its definition varies greatly from party to party, though everyone believes that everyone else's definitions are wrong. It is generally agreed to be something possessed by every person, and that's about all. Therefore, the soul may or may not be any of the following:
1. The human mind, that is, that thinking thing lodged behind your eyes;
2. The essence of humanity;
3. The essence of that which makes a person good and decent;
4. The quality of sentience or
human intelligence;
5. All of someone's personality or what makes them unique;
6. Some mystical version of a person that lives
on after the body dies;
7. A spiritual concept, created by God (or the gods if you prefer) or a part of him;
8. The quality of
being alive;
9. Anything else you can think of along these lines.
Note that it is also up for debate whether or not
non-human animals, or for that matter, plants, have souls; feel free to adjust these answers accordingly.
Strangely, no one seems to suggest that
animals have souls and
humans lack them.