What makes a C area a "C"? Or A, B, etc???

[Greg Hamer]

While I am familiar with good and bad areas in our neck of the woods, I am curious as to the criteria that is used to rate neighborhoods?

What makes it an A?  Nice cars, no trash, big homes?

Or a D?  Cars on jackstands, bars on the windows, crap everywhere?

Any insight?  What does everybody else use?