What are ghosts?

I'm still trying to get a handle on my ghost story.

What does a ghost story mean?  Are ghosts bad memories, representing the past?  Are they vengeance and justice?   Are they just free-floating evil?  Are they from some portal into darkness?  Are they unrequited life?

I need to get a sense of what the book is, somehow.   Because I don't have a clear sense of what I'm trying to say, I haven't started yet, after a week of mulling it over.

But I've waited long enough, nothing has happened.  So I'm going to go ahead and start writing my 2000 words per day, starting today, and hope that something starts to develop.