Ghosts seem to be a catch all phrase for many things these days- I am not certain that I am going to believe that they can't be a person that has left their once living body, maybe they (we) can and do. But like princepts, I have had a few very weird experiences that make me certain that there could very well be something else going on. I am not so sure I would just believe any "thing" that was telling me it was my dearly departed family, or god or anything of that nature. It is very possible that there could be things that we do not know about that have the ability to move about and trick us into thinking they are the dead. I certain would approach any instance like that with the upmost caution and open mind.
I remember a few years back when ghost hunting seemed to be such a rage, there were folks that just wanted to have the experience, and I would say time and time again, that you just could not be sure what it was that you were really experiencing. It could be aliens, it could be demons, it might even be something inter-dimensional, but for the most part I am certain that a lot of it was imagination.