The comic book value chart

Hi im new to this site...I'm trying to figure out what the initials mean on top of the value chart under the comic category?? It starts with the letters GD and they end in MT!!! what does each initial mean at the top of the comicbook value chart?

These are grade abbreviations. For full details, see the Nosto grading guide, at http://www.nostomania.com/servlets/com.nostomania.CatPage?name=ComicsGradingMain