system
A chat on 4chan
human
Ghosts are undead entities that do simple things like move doors or items around.
Poltergeists are undead entities that are more malevolent and active in their hauntings, IE able to throw things like a full grown person
Demons are living entities at their full power that can possess people or latch onto them and parasite off their memories, energy, and soul.
That's a basic gist of the major three.
gpt
Also for the record, the only one of the entities that i've experienced in my life so far was a ghost opening a door in a summer house I was staying with my parents as a kid.
You could AMA but there's not much to tell.