Ghosts are spirits which linger after death from being aware of emotionally painful and/or traumatic ends. Most are mindless, but the stronger-willed retain some semblance of their former selves. However, their minds are usually warped to a great extent by their deaths, being fixated upon the thing (or person) which caused their demise. They tend to dissipate over time unless they can find some living, human, mind to sustain them, generally that of someone who was close to them in life. This gives rise to possession (for ghosts which retain their minds) or poltergeist activity (with mindless ghosts). The only signs of this are the victim having occasional screaming nightmares, occasional sleepwalking, being the centre of ghostly/poltergeist activity, and (sometimes) developing skills which the ghost possessed in life.
However, possession depends on the ghost having a significant Psyche advantage over their victim, and also on the victim remaining unaware of their possessed state. If the victim has (or develops) a Psyche comparable to or greater than the ghost, then it may be unconsciously forced out. If they become aware of the ghost then (even with a lesser Psyche) they can, at least temporarily, force the ghost out.
The power of a ghost partly depends on the Psyche of the original person and partly on how traumatic their death was (the more traumatic, the greater the power).
Ghost have some link with Tir-Na Nog'th; parts of it may form a visible aspect of the land of the dead - but they are only visible in the full Moon.
Most magic cannot affect ghosts, due to their dead nature. Some kinds of magical necromantic and summoning rituals, plus non-magical seance-type rituals can also contact them. These can all (with enough power) bind or destroy them, but have the disadvantage that they must be done in the same location as the ghost and can only affect pre-existing ghost via lengthy rituals. Normal Trumps of the ghost in life do not work. A path of Trump Voodoo can contact and control ghosts, but this is a very rare ability; it does have the advantage that it will work from anywhere (though ghosts may not be very coherent, communicative or co-operative, of course). The Jewel of Judgement can also bind or destroy ghosts.
While possessing their host the ghost can take over their bodies while they sleep (not if they are unconscious), and act within them. In this case, the possessed person takes on the statistics of the ghost rather than the person. So, if the ghost of a strong person takes over a weakling, the weakling will have the strength the ghost possessed in life.
Some places have reputations as being especially haunted. House Karm, in Amber, is one; because of this, House Karm has a number of professional exorcists among the family Karm or on its staff (this is not something which is too generally known). Places of imprisonment (e.g. the Amber dungeons) and execution are others.
The powers of ghosts vary, but can be any or all of :
CONTROL MATTER (telekinesis/poltergeist activity); Strength (which can be split among many smaller objects) given by the ghosts Psyche, accuracy given by its Warfare; the bigger the object, or the more objects, the less fine control the ghost has. It is invisible to sight, but Chaos or higher Psyche will always detect it. Amber Psyche gives an effective Strength of Human ranked; Fiona's Psyche would allow the Ghost to lift tons. Intelligent, malevolent ghosts can use this to attack others; mindless ones use this simply for poltergeist activity.
CONTROL THE ENVIRONMENT (chilly, strange noises etc.). This cannot physically harm people, but does worry them. This overlaps a great deal with the control matter ability, above.
INDUCE/CONTROL EMOTIONS (in an area); this depends on the relative Psyche of ghost and others; mindless ghosts may simply project whatever they feel (rage, pain, sadness) in an area; intelligent ones may actively try to control the emotions of others to their own ends.
CONTROL HOST (usually when the host is unaware; even when aware ghost may be able to make the host talk with ghosts voice, or freeze host in place, though probably not both at the same time). A particularly powerful ghost might slow the host while talking through them.