I've been fascinated by ghosts for decades and I from what I've heard, there are a few different types. There is of course the most common type which is the earthbound spirit of someone deceased who for whatever reason has decided to "haunt" a place that they were familiar with when they were alive.
Then there are the evil spirits that attack people and actually cause physical injury in the form of bites or scratches, the most famous case being the one which was made into a bestselling book and a movie called "The Entity". Those can be either mischeivous spirits of deceased people or they can be actual demons.
The rarest type is the helpful or warning ghosts who actually help people or warn them of things and the people don't realize it was a ghost until after the fact. In South Carolina there is a famous ghost of a fisherman called the Grey Man ghost who walks the beach and legend has it that whenever he is seen it means a hurricane is coming within a few days and that whoever sees him will have their house spared. I saw an interview with a couple who saw the ghost and thought he was just a gentleman walking on the beach in a fisherman's outfit. A hurricane came in a few days and this couple's house was unharmed while the homes next to them suffered severe damage. The most interesting part is that they videotaped these towels that they had hanging on the railing of their porch and the towels barely moved even though you could tell that the wind was blowing fiercely.