A ghost is the spirit of a person who has died but has not been collected by a Reaper. The main reason for this is often a refusal to pass on due to either some form of unfinished business, or a fear of what awaits them on "the other side". Most ghosts shift between wandering the mortal realm and a place known as The Astral Plane.
The Plane overlaps greatly with the mortal realm, and oftentimes a ghost will wander in a place of significance to them, such as a home they lived in. Other times, they will latch onto an individual they feel a connection to. When a ghost fixes itself to one spot, be it a building or a person, it is known as a "passive haunting". If the ghost tries to or succeeds in making itself known to those around it, it becomes an "active haunting", and if it tries to harm people or property, it is listed as an "aggressive haunting".
Ghosts cannot actually speak, though they can interact with the world, typically by manipulating objects in the surrounding environment. In instances where they wish to convey a message to someone, they may try to leave a recording on a device, known as an Electronic Voice Phenomenon, or EVP. These messages are often hard to hear clearly, even with the best recording equipment.
Ghosts can only be seen by Angels, Demons, Reapers, and other immortal beings. For a mortal to see them, they must travel to the Astral Plane and return safely. The page for the Plane details what happens in that realm.
Types of Ghosts
The term "Ghost" is both a blanket term that covers several types, as well as one of the types covered. A "ghost" t has yet to be collected by Reapers, and therefore, is still tied to the mortal world. A "spirit" has been collected by Reapers, and thus, has no connection whatsoever with the mortal world. They are either hunting in Purgatory, burning in Hell, resting in Heaven, or, in rare instances, returned to the Astral Plane. Finally, there are "poltergeists", which are ghosts that have slipped into insanity and become hostile to mortals, attacking anyone around them, including other ghosts.
However, if one is indisposed in one of the afterlives, then how does one have time to contact the living who use Ouija boards or employ the powers of a mystic to invoke their name? Simple: They rarely do. The desired spirit is rarely the contacted one. This has led to situations where poltregeists, or violent spirits, decide to wreak havoc on the people who contact them by mistake. Stories abound of families plagued by restless spirits that were contacted and decided to keep contacting the family.