Ghosts or spirits are believed to be the human spirit or life force of those that once inhabited a physical body here on earth. Psychics claim to talk to them, and ghost hunters encounter them, at least through photography and other electronic means. It is said that they are those spirits that have willingly stayed behind because they feel bound to their previous life or previous location for whatever reason. Because of this obsession they tend to become similar to a psychotic human being. It should be said that the majority of us when we die proceed gladly and willingly to the next level of existence after saying our quiet good-byes, which means we're off to the spirit world. Then again, as stated, a select few elect to stay behind because of a refusal to move on. Apparently the longer they stay behind, the harder it is for them to move on, which again, is the spirit world.
If you're wondering, ghosts are normally harmless for the most part and have relatively no desire to bring harm to anyone nor do they have much of an awareness to act as such. Ghosts however, have been known to disrupt the lives of those that they affect and play pranks and move things about. In actuality, they are limited in the activities that they are able to carry out in their form of existence.