The Astral Plane is the source of Magic, and intersects the physical plane at Leylines. It can also be visited and even used as a means of travel, though with great difficulty. Beings on the physical plane must have an astral-form to use magic, and very powerful mages can draw energy directly from the astral plane. There are very few creatures that exist in only one form or the other. This is one of many Planes of Existence.