You know, Vanessa, that is a good question. I have been a born-again Christian for over 30 years now, and the whole "ghost" issue is perplexing.
I do not think that ghosts are the spirits of people who have been trapped here and cannot move on. I believe when a person dies, anymore, that they go immediately to judgment.
So, I think ghosts can only be one of two things. Either they are heavenly spirits or demonic spirits. Since heavenly angels do not parade around as your great grandfather in an old attic and since they do not cause fear in people unnecessarily, then they must be demons who manifest themselves as the dearly departed to cast the illusion that spirits can survive death and judgment.