Well it seems self-explanatory.
Taking hold of a body in the means to either consume it indefinitely or to control it and the actions. It’s a very high level power and rare enough to find and has a very high price on the black market.
Possession seems to suffocate the true body owners soul, tarnishes it a bit with the actions of the demon possessing it.
Sensationally speaking it is very much losing control of the body the actions often negate the intention of the original soul.
That being said memory is not always left intact after possession is released often causing black outs and confusion.