According to traditional belief, a ghost is the soul or spirit of a deceased person or animal, taken to be capable of appearing in visible form or otherwise manifesting itself to the living. Descriptions of the apparition of ghosts vary widely: The mode of manifestation can range from an invisible presence to translucent or wispy shapes, to realistic, life-like visions. The deliberate attempt to contact the spirit of a deceased person is known as necromancy, or in spiritism as a séance.
The belief in manifestations of the spirits of the dead is widespread, dating back to animism or ancestor worship in pre-literate cultures. Certain religious practices—funeral rites, exorcisms, and some practices of spiritualism and ritual magic—are specifically designed to appease the spirits of the dead. Ghosts are generally described as solitary essences which haunt particular locations, objects, or people with which they were associated in life, though stories of phantom armies, ghost trains, phantom ships, and even ghost animals have also been recounted.
Another widespread belief concerning ghosts is that they are composed of a misty, airy, or subtle material. Anthropologists link this idea to early beliefs that ghosts were the person within the person (the person's spirit), most noticeable in ancient cultures as a person's breath, which upon exhaling in colder climates appears visibly as a white mist. This belief may have also fostered the metaphorical meaning of "breath" in certain languages, such as the Latin spiritus and the Greek pneuma, which by analogy became extended to mean the soul. In the Bible, God is depicted as animating Adam with a breath.
In many traditional accounts, ghosts were often thought to be deceased people looking for vengeance, or imprisoned on earth for bad things they did during life. The appearance of a ghost has often been regarded as an omen or portent of death. Seeing one's own ghostly double or "fetch" is a related omen of death.
White ladies were reported to appear in many rural areas, and supposed to have died tragically or suffered trauma in life. White Lady legends are found around the world. Common to many of them is the theme of losing or being betrayed by a husband or fiancé. They are often associated with an individual family line, as a harbinger of death. When one of these ghosts is seen it indicates that someone in the family is going to die, similar to a banshee. Legends of ghost ships have existed since the 18th century; most notable of these is the Flying Dutchman. This theme has been used in literature in The Rime of the Ancient Mariner by Coleridge.
A place where ghosts are reported is described as haunted, and often seen as being inhabited by spirits of deceased who may have been former residents or were familiar with the property. Supernatural activity inside homes is said to be mainly associated with violent or tragic events in the building's past such as murder, accidental death, or suicide — sometimes in the recent or ancient past.But not all hauntings are at a place of a violent pass, or even on violent grounds. Amongst many cultures and religions it is believed that the essence of a being such as the 'soul' continues to exist. Some philosophical and religious views argue that the 'spirits' of those who have died have not 'passed over' and are trapped inside the property where their memories and energy are strong.
Grabbed from the Wikipedia!