I am not a professional grader, however I follow to the best of my ability, the grading guidelines set by the standards set in this guide: http://comicbooks.about.com/od/buyingcomics/p/howtograde.htm

In general, a NM+ comic is one that I cannot personally find any flaws or faults in.

NM is one that is in great condition, but minor creases on the cover or white marks on the corners.  (Not quite blunting, but wear)

VF is one with slightly more wear than a NM comic book.  May have blunted corners, or white marks throughout the spine indicating that it has been read before.

FN is usually a comicbook that is older, showing signs of being read before, but in general well preserved.  Will likely have yellowing pages, and maybe a folded corner.  Perhaps small holes (less than 3 millimeters) from age on the inner pages.

VG is a comicbook that is obviously used, but still nice to look at.  Multiple folds are likely, and maybe even a tear.  Still readable.

GD is a reader copy.  One that wouldn't be worth keeping as a collector, but great for those wanting to enjoy a book rather than just look at it.  (You know who I'm talking to, you collector who's never even opened your books.  LOL)