In short, they are your soul. The spelling is taken from the His Dark Materials books. The concept in the books was taken from Aristotle's concept of a daimon who was his spiritual guide.
A daemon in our world is incorporeal, more of a voice in your head. When people visualize them, they are usually animals. They can shapeshift to almost anything. Eventually they will settle as an animal that reflects your personality and character the best.
When a daemon takes an unusual form, usually for self confidence. The forms are usually wolves, wolves with wings, strange hybrids etc. Not everyone with a wolf, mythic, or hybrid daemon is an uberwolf, though.
Sara's daemon's old uberwolf form was a lion with eagle wings and spikes.