DAN'S QUICK AND DIRTY COMIC BOOK GRADING GUIDE
THE SYMBOLTHE SYMBOL SPELLED OUTWHAT IT ALL MEANS
PR POOR Means just what it says. This comic is pretty much trashed and uncollectible. Might even be missing a page or two. The only exception is coverless comics. A coverless comic can be in fabulous condition but is still graded as Poor due to the fact that it has no cover.
FR FAIR Means "not fair at all". This is a comic that is just hanging in there. They are usually either really beat up or have some serious issue like a missing back cover, big chunk out of a cover or a coupon clipped from a page. The key here is that this is still a complete and readable comic. Often referred to as "readers".
GD GOOD Means "not so good". This is a low grade comic that has seen a lot handling in its day and looks and feels rough. The comic is complete and readable though it might have a small piece (we are talking ½ inch here) out of the cover. Collector's will hang onto one of these for only as long as it takes to find a better one.
VG VERY GOOD Means "okay". This is a little below average for a used comic. Shows a fair amount of normal wear and tear with some creasing, loss of color, etc. but nothing too major. The comic is complete with maybe a tiny piece missing (1/4 inch).
FN FINE Means "not too bad". This is a little above average for a used comic. Looks and feels pretty nice but has enough minor dings like small creases and such to keep it from grading higher. Complete, readable, presentable.
VF VERY FINE Means what it says. This is a pretty sharp comic. Has some minor issues but they are minor. Might have a small crease but it cannot break the color for example. Picky stuff like that.
NM NEAR MINT Means what it says. This is your basic Mary Poppins comic. "Practically perfect in every way." It can have some very minor issues but you have to search to find them. If you look at a comic book and ANYTHING negative jumps out at you the chances that it is Near Mint are just about zero. Older comics (pre 1970) are very difficult to find in this grade - and they generally cost a ton when they do show up.
MT MINT Means absolutely freakin' perfect! Other than brand new unread modern comics, good luck finding one. There is a rumor out there that they exist but Elvis supposedly has all of them.

Keep in mind that it is much more complicated than this. Some things, like "tweener" grades (FR/GD, VG/FN, VF/NM, etc.) are pretty much self-explanatory. Other issues that can affect a comic's grade are much more technical. There are plenty of resources on the Internet if you want to explore comic grading in more detail but at least this quick list will give you the general gist of it.