I have watched quite a few Animes where they rank certain characters but never seem to explain what these rankings are. Does anyone know of a website or have any information on what these class ranks represent?

Here are the ranks I often see:

A Class
B Class
C Class
G Class
S Class

There might be other classes, and if you know, please let me know. An example of some series that use them are Kiddy Grade and sCryed, but I know that many others use it, like S-Class criminals in Naruto.