Ghosts are believed to be the spirits of dead people, that for some reason stay on after the person has died. Some believe that the spirit is unable to move on because the spirit has some business in the world, that has not been done yet, such as avenging a crime, or similar. In some cases, ghosts are reported as menacing. In other cases, ghosts may be said to not appear scary or malevolent at all, but can be almost a friendly presence.
It is reported that what is believed to be ghosts or spirits may be able to affect the physical world, for example by creating knocking-sounds or by moving objects. This type of phenomenon is known as poltergeist (which is a German term that literally means knocking spirit).
Some say demons are also a form of ghosts, but then the spirits of some beings other than human.