A spirit animal is a spirit that embodies all of the characteristics of the species it represents. It is usually not viewed as the spirit of a specific animal, but as an archetype of that animal.
Many individuals claim a spirit animal as a personal guide for their path or life or simply to aid them through a specific phase in their life. In this case, the spirit animal may be referred to as a power animal, a spirit guardian, a spirit guide or a totem. People select spirit animals (or are chosen by them) with whom they identify or who exemplify traits they wish to cultivate in themselves or who can aid them in learning those lessons set before them in this lifetime.
As with just about any topic related to magic and magical spirituality, the fact that a spirit animal is defined differently in different traditions makes it somewhat difficult to define, but generalizations can be made. You the reader are encouraged to chime in with your thoughts, experience, wisdom, and opinions on this subject.