Possession is when someone or some being is overshadowed by an Other of some kind. This happens through either a craft of some kind, such as Shamanism or when someone bleeds out enough to leave a gap open enough for something to fill in the void. When this happens the body will try to reject it, but if the invading being can get a solid foothold they can take control or influence the host.
Victims of possession can gain benefits from it as well. To heal, to be stronger or faster, it depends on what has possessed them. A possession where it cannot control the host is a light possession.