Grading System:

M (mint)
Record is new, shiny, never played; cover is new, sometimes record is still sealed

NM (near mint)
Record is like new, probably played a few times, but that doesn't effect on the quality of a sound; cover has none lack, it is almost new

EX (excellent)
Record in great shape, may have light marks, played more than a few times but low in surface noise or no surface noise; cover may be little bend on corners and edges and have a little fade in colors

VG+ (very good plus)
Record has been played more times, it has marks and surface noise, it doesn't skip; cover has fade in color, and has more damage on the corners or edges

VG (very good)
Record has been played many times, more surface noise, but it shouldn't skip while playing; cover has more cuts on corners and edges and more fading in colors

G (good)
Record or cover are in bad condition, this is worth only for archive items - just to have that until you find better one, but please be aware that first pressings are very well recorded, especially singles, so good condition can sound much better!