Anyone who investigates the paranormal realizes that there is something out there. Exactly what that something is can differ from investigation to investigation. Spirits are essentially energy, that is why you are able to pick them up with a K-II or a MEL-Meter. It is the different types of energy/spirits that make each case different.
Ghosts are spiritual energy that was once attached to humans or animals. They retain the personality characteristics that they had in life. Generally they are trapped between realms, unable for whatever reason to cross over into the spiritual realm. Generally they are harmless, but of course there are a percentage that were evil in life and remain the same in death. Many cases they are residual, just like a tape playing over and over again. Some are one time appearances. Your uncle is in the hospital, you turn in the kitchen and he is there smiling, then gone. You then find out he died. These spirits can of course cause problems, but can only levitate small items. Generally they are not trying to scare you. They are trying to exist, make contact or are not even aware you are there.
You also have hungry ghosts, ghosts that could not get what they needed in the afterlife. They come back to feed off of humabs fear energy. These are commonly found in Taoism. Child ghosts are particularly upsetting. They generally are lonely and looking for attention. Animal ghosts are animal energy attached to a location. You can see the previous owners cat running in your front room. Vengeful ghosts are to be avoided. These are ghosts that attack men or women because of perceived indiscretions caused by them in life. These ghosts attack humans, by scratching, choking or pushing.
Spirits is a term many use to describe entities that were never human. Angels, demons or elementals, all of wich are not to be trifled with. Angels are exactly what you expect, they are good, can be asked for help and sometimes will. But again they can cause harm if trifled with. Guardian angels are human spirits that watch over a specific person. They try to protect them. Animal guides are essentially the Native American guardian angel. They try to guide the person to do the right thing. You don’t call on an inhuman spirit, you never know which one you will get.
You then have Elementals and shadow people. Elementals are attached to the land; protecting it from harm. Normally they will not harm you, but will if they must. They use fear to move people off their land. Some see them as minor gods, druids worship the deities. To appease these spirits an offering of some kind must be made, in many cases that will work. Shadow people, not much is known about them. But as cameras improve and shadow detectors are used we are gathering more evidence. Some people see them as malicious entities, others think of them as watchers from another realm. Either way they are very off putting and make me uncomfortable.
Of course we have the Demonic. These are the spirits I believe most paranormal investigators wish to avoid. These spirits are an affront to God and mean to destroy humans. There main goal is possession and death. They are evil, intelligent and have a hierarchy. They get more powerful as you head up the laddet. Generally if there is one demon there are more. Some quick telltale signs are smells, sulfuric and rotting, taps or knocks in 3s, teleportation of objects, amongst several others things. Many times they were invited in and others they were already there. Unlesd you are a shaman or an exorcist you should not challenge a demon. These beings are feared by most ghost hunters and one famous group had a smart saying. “When in doubt, get the hell out.”
This is just my experience and what I have read. If you have different ideas please feel free to add or correct. This is not an exact science, for me it has been trial and error. Always looking for more input.