Chaos Magick is one of the newest arrivals on the scene. It was first conceived in the 1970s. It draws strongly from Crowley and Austin Spare (a contemporary of Crowley's who worked a lot with sigils).
Essentially, Chaos has no gods or goddesses and sees itself as being the framework on which all other Magick is based (i.e. the "mechanics" of Magick). A central theory in Chaos Magick is the Magickal power of the unconscious mind. It is held that if an event is willed by the unconscious mind, it will occur as long as the conscious mind doesn't get wind of what's going on and put a stop to it! The explaination for how this occurs usually involves complicated and (sometimes slightly dubious) quantum physics as well as chaos theory: Every event is thought to be linked along the lines drawn up by conventional chaos theory, and the unconscious mind is supposed to understand these links and perform the neccessary actions to result in the willed change.
Chaos Magic puts a lot of emphasis on the individual practitioner discovering ways of working for themselves rather than blindly following accepted dogma. Chaos rituals are often very chaotic!