What's an A, B, C neighborhood?

[Jaclyn B.]

I posted the question below in another forum but it didn't get answered. Still trying to figure out what goes where. 

Can someone define for me the difference between A, B, C, D neighborhoods? I've seen these terms used throughout the forums and I'm just curious how people define these different types of neighborhoods.

Intuitively I understand the difference between an "A" neighborhood and a "C" neighborhood. But what's difference between "B" and "C"? Any input is greatly appreciated.

Jaclyn