A power animal is an animal who's characteristics have what you lack yet desire. Animals you are drawn to are likely to be something you lack within yourself. People can change power animals as they age and change. Or for what the situation calls for. A shaman who wants to learn to be quiet may learn the ways of a mouse.
An animal guide is an animal that relates to you closely. They may guide you physically (by learning more from their species therefore learning more about yourself). The animal guide or spirit animal are a native american shaman's best friends. They travel with them where ever they go, Help them by leading them through astral journeys, and aid them in magick (like the european witch's familiar). Animal guides rarely if ever change. But they can if the person experiences an extreme change of personality.
My power animal of choice is the raven.
My animal spirit is the panther/cougar.