Ghosts are an endlessly fascinating topic. They go way back in history. There are numerous records of sightings of them. They have made their way into literature, television and movies.
Yet there is no way that traditional science will accept their existence since ghosts cannot be "proven" in a manner which meets the demands of scientific proof. They cannot be brought into the lab. They cannot be specifically measured and quantified. They cannot be summoned to appear on demand over and over. Thus science considers them not worthy of actual study.
Yet there have been so many sightings that the old adage "where's there's smoke there's fire" seems to be reasonable. There are numerous types of ghosts and apparations. Their reasons for being here can be quite complicated. Whether or not they actually represent some form of actual intelligence is still up to debate and may never be answered in a final manner.
----------Books About Ghosts----------
Unsolved Mysteries episodes about ghosts