Why does the back brakes grind like sand paper?

I had this same problem on my 1994 Pontiac Bonneville. It sounds like a horrible scraping sound that shouldn't be there. Sometimes this can be caused because it has not been in use for a while, or if it has just recently rained. Mine kept doing it continually even if it didn't rain or I just used it recently, so I took it into the dealership my dad works at. It turns out that my brakes had some problems. So really it could be either of these things. My guess is that it is just because it has been sitting for a while. I'd say drive it, and see if the should gets any better, if it doesn't, take it in to see what is wrong. Hope this helps!