Ghosts are, as you probably already know, the lingering spirits of the deceased. There is plenty of debate on whether they are truly the liberated souls of those who have passed from this world or if they are, instead, merely the lingering energy of those people. It is my opinion that both theories are likely to be correct in some fashion.
The tricky thing about ghosts is that there are no discernable rules when dealing with them. Like when they were living, each one is a unique individual and no two are ever exactly alike.
Spirits can manifest themselves in vastly different ways. Most of them are little more than shadows of their former selves, and seem to be doomed to replay small, mundane events from their lives over and over again for all eternity. These residual spirits make up the vast majority of all known spirits, and it’s unknown whether they are true spirits or merely reflections of the dead. Other ghosts seem capable of a minimum level of interaction, but are too weak to manifest themselves completely.
Still other spirits, however, are nearly indistinguishable from the living. They appear solid and firmly grounded to this world, they converse clearly and seem to know where and when and what they are, and they can even interact seamlessly with their environment. It is impossible to say how many of this kind of spirit exist in this world, as very few people ever realize they aren’t interacting with a living, breathing person. It is rumored that you can tell the difference if you touch one of them, but they are notoriously evasive when it comes to physical contact, which I suppose only makes sense. The only way to be sure is to take a picture. Ghosts don’t photograph normally and will usually appear as blurs or mists or other flaws on the film.
All of these aforementioned types of spirits are generally harmless. Only a few hostile exceptions have ever been recorded. However, there are several types that are always quite dangerous.
A true poltergeist is extremely rare, but devastatingly real. It rarely manifests itself physically and channels its energy instead into its environment. It is usually attached to a particular place or object and can be a nightmarish experience for anyone unfortunate enough to find themselves taking the brunt of its rage. Once angered, it is extremely difficult to escape. Unlike other strong spirits, the poltergeist never takes a specific, physical form.
Another dangerous manifestation of a spirit is the dark or cursed spirit. It shares the poltergeist’s violence and rage, and may in fact be the same kind of ghost. But whereas the poltergeist is defined by its lack of a physical manifestation and its habit of manipulating its environment to terrify its victims, the dark spirit fully manifests itself, often taking a horrific and murderous form. Cursed spirits seem to have been around a very long time, since they often appear in long-deserted and long-forgotten places, making it possible that they have become what they are because of their endless, pent-up rage.
On rare occasions, a ghost can also manifest itself as a monstrous, inhuman form, making it difficult to even identify as a spirit and can often be mistaken for a demon or monster. They can also sometimes take control of vehicles and computer systems.
Finally, there is the shade. Shades are spirits of the dead that are manipulated by dark and powerful forms of magic or incredibly advanced psychic or spiritual abilities. Shades are often ripped straight from their graves and strongly resemble the person they used to be, but very little, if anything, remains of their humanity in this state. They are little more than puppets. They should not be underestimated. Although seemingly mindless, they can be used to commit extreme acts of violence and have been known to horrifically dismember their victims on the command of their puppeteer.
Additional: It’s possible that the length of time a spirit has been around might have an impact on its strength. Recently deceased spirits might often be weaker than those of people who died centuries ago. It’s also possible that there’s a relation between a spirit’s strength and the age at which the person died. But it’s difficult to prove such theories, since ghosts are, by their very nature, elusive.