Astral Possession is the ability to take control of the body of another being by leaping into that body in astral form. Once the user has possessed another being, they gain full control over their body and powers. However, they will not be able to control their minds and do not have access to their memories.
Astral Projection is the ability to project the consciousness outside of the body. This power has two basic forms, one where the consciousness takes a corporeal and visible form and one where the consciousness is projected in an invisible astral form.
Possession is the ability to control other beings by entering their bodies. While possessing another being, the user assumes control over said being's body and powers.