A vs B vs C vs D neighborhoods?

[Aaron W.]

New real estate investor looking for properties.  How do you know the difference between an A, B, C, and D neighborhood?  I believe an A neighborhood is pretty straightforward as a high end, expensive.  A D is what I have heard called a "war zone".  

How do you define the difference between these neighborhoods and how do I do a search for these different neighborhoods without physically being in these cities if I am doing an initial real estate search (i.e Google Maps, etc.)?