Before looking at the theories that have been proposed to explain ghosts, it is useful to clarify how researchers classify the different types of ghosts. Those ghosts, the fact of observation of which is confirmed by any circumstances, are called "reliable" and are of the greatest value for parapsychologists.
Many of the researchers whose interests lie in this area have developed their own classification for themselves.
The following are the main categories considered by the various societies of mental researchers.
"Crisis" ghosts
In this case, we are talking about ghosts, which are at the moment the agent is experiencing the highest crisis (especially before death). In most cases, such ghosts appear in front of those with whom the agent has the closest emotional bond. Apparently, most of the "crisis" ghosts are intended to inform living people that the agent is dying or has just died. Some of them even make gestures to indicate their mortal wounds. "Crisis" ghosts appear both in waking dreams and in normal dreams.
Ghosts of the dead
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In most such cases, we can talk about the presence of close emotional ties between the observer and the person in whose appearance the ghost appears. Apparently, here the goal is to bring peace to the hearts of the mourners or to communicate some information regarding the will or some unfinished business by the deceased. For example, after his death, Dante appeared to his son and pushed him to search for secret places where the last songs of his "Divine Comedy" were hidden. The son knew nothing of their existence. The ghosts of the dead can appear even years later, appearing at some crisis moments in front of living people who loved them.
Collective ghosts
Most ghosts, both living and dead, appear before percipients when they are alone. "Collective" are those ghosts that are observed by several people (at least more than one). Such cases were noted both among the ghosts "tied" to one place, and among the "crisis" ghosts. Approximately a third of all described cases belong to the category of collective ghosts. It should be borne in mind that the group of percipients may also include animals, according to the reaction of which it can be argued that they noticed something unusual. For example, a dog may whine and hide, and a cat may arch its back.
Mutual ghosts
This category includes ghosts of living people, and the agent and the percipient seem to see each other. In most cases, this kind of agent has a great desire to be near the percipient (this desire may be due to a feeling of loneliness, longing, love, or anxiety). The agent suddenly finds himself in close proximity to the percipient, who, in turn, is observing the agent. Mutual ghosts can be collective at the same time. One possible explanation for this phenomenon is that the agent can "project" from a state that allows for a sense of detachment (see Wilmot ghost).
Deathbed ghosts
There is not so little evidence that people who are on the verge of death are angelic creatures, various religious figures, luminous outlines and images of loved ones. At the same time, in some cases, ghosts were also observed by those who looked after a dying patient (see Visions on the Deathbed).
Ghosts hinting at reincarnation
Cases of a kind of “prophetic dreams” were also reported, when the deceased appeared in a dream to a member of the family in which the deceased person was destined to be born and thus receive his new bodily incarnation. Dreams of this kind are very common among the Tlingits and representatives of other tribes of North American Indians living in the northwest of the continent, as well as among the Turks, Burmese and the people of Thailand. It is known that sometimes in Burma or Thailand a child discovers some scraps of memories of his previous lives and says that he remembers how he sent his future family a "prophetic dream" about the upcoming appearance. Sometimes (though quite rarely) a child may appear as a ghost of his future mother (see Reincarnation).
From the book: "Encyclopedia of Ghosts and Spirits"