1. At the center of the circle is placed the altar, which magically represents the center of the universe and the center of the self. It is the working surface of ritual, the focus of awareness and power. When there is no space for a material altar, one may be visualized in the astral. The altar is square on top, cubic or doubly-cubic in dimension, and should be made of, or contain, natural stone. All these aspects symbolize that the altar is the material foundation that supports the work; it is the fixed place, the earthly buttress, where power is manifested and made actual in the world.
SOURCE: The Truth about Ritual Magic, Donald Tyson
2. An altar is really a work bench holding your magickal tools. Usually it is placed at the center of the circle or working area. Sometimes it may be moved to face any of the four corners. Traditionally it is composed of two equal size cubes, one on top of the other.