When a spirit gets "stuck" on this earth plane, they are referred to as Earthbound. However, they are not really "bound" at all. Each of them can choose to move on at any time. Some of them choose not to, as described above. But others are truly lost, and do not know that they are dead. Spirits of this type usually get attached to a certain place (like the house they lived in at the time of their death) and refuse to leave. If you have the misfortune to move into the house, the spirit will likely not take kindly to you being there! Remember that they think it is still THEIR house. How would you feel if some strangers suddenly moved into YOUR house? I bet you wouldn't be too pleased!
Spirits will often try getting your attention by speaking to you, tapping you on the shoulder, or even throwing things. This does not mean that you have an "evil" spirit on your hands. Truly evil spirits are very, very rare. (I will talk more about those momentarily.) Rather, you have the spirit of an unhappy person on your hands. They feel you are invading their territory, and of course they will try to let you know that they are unhappy about it. Sometimes though, they aren't even unhappy about you being there, they just try to let you know they are there, period. They may cause certain physical phenomena to occur (moving objects, voices, apparitions, flashes or trails of light, shadows, footsteps, doors opening and closing, etc). Remember that it can get quite lonely being a ghost. Imagine spending years without anyone knowing you are there. Most people can't see or hear you. Then one day someone comes along that senses your presence, and you know it. Of course you'd try to let them know that you are really there.
Good Ghost, Bad Ghost
Remember that when you die, your general personality does not change. If you were nice in life, you will be nice in death - and vice versa! It is very possible to encounter a loving ghost, as well as a mean, nasty one. But again, mean and nasty does not mean "evil." Evil spirits are typically not human; they are demonic. As I said above, they are extremely rare, and most people will not encounter them in their lifetimes. What most people refer to as an "evil spirit" is really just an unpleasant, or dark, human spirit. The difference between dark spirits and evil spirits is this: evil spirits are usually demonic in form, not human. They are evil because they are not "of God." They were not created by God. They are spiritual manifestations of darkness, destruction, chaos and suffering. They serve only one purpose: to create more darkness in this world. Dark spirits are typically human, but they are distant from God. They were created by God as Light beings (like all of us), but then transitioned to darkness and they are now separated from God. (Either by choice or circumstance) But they always have the choice to return to the Light.
How can you tell the difference between an evil spirit and a dark spirit? Dark spirits usually feel very cold and "dark" (as in, not good), but they do feel human, and may appear as human if you see them. You may feel frightened of them, or even threatened, like you need to leave the area immediately. Evil spirits, on the other hand, may be accompanied by a horrible, rancid stench. (not always, but often) They *emanate* evil. You can really feel it, it's a black, dank, empty feeling that seems to soak right into you. Believe me when I tell you that you wouldn't easily forget it if you'd ever come face to face with one of these beings (and most of us never will).
It can be difficult to tell the difference between truly evil spirits, and those that are just "dark," because our fears can be triggered even by dark spirits, making us think that we are dealing with an evil spirit.