It is best to start this stating what ghosts are not. Ghosts are not souls, they are not spirits. Ghosts, by and large, ARE apparitions. That is to say, psychically and emotionally transduced echos of beings that are no longer present physically and are most often dead.
it all depends on your perspective of the afterlife, and, in some cases, religion, as some religions believe that ghosts are evil and are the spies/demons of Satan. I believe that there is some form of afterlife or a world parallel to ours(such as "heaven"), and that ghosts are spirits of the deceased or mere replicas of their bodies living in our world, though they do not know that they are dead.(hence why the try to communicate with our faces.