Astral Projection is when one projects ones consiesness outsite of their body. While doing this you are in what is known as The Astral Body. While in this state you exist on a different plane of existance: The Astral Plane. For tips on astral projection see Techniques of Astral Projection
Astral Projection is achieved usually when one has entered a state of complete focus. While Astral Projecting one can do things considered inpossible in the physical plane. For example, phasing through solid objects, flying and teleportation.