Mint unopened / un-played, often still in shrink-wrap or has not been removed from the jacket.  

The sleeve is pristine condition, looks new.

NM (Near Mint) very light signs of play, perhaps fine micro-lines from being removed from sleeve. 

The sleeve may have very light corner wear, the odd small dent / line.

VG+ will show some signs that it was played and otherwise handled by a previous owner who took good care of it. Defects should be more of a cosmetic nature (slight scuffs, very light scratches or slight warps) that don't affect one's listening experiences. Sleeve will show some wear such as light creases but will be in-tact.  There may be light ring wear / discolouring

VG  marks or scuffs may be felt by finger-tip touch, visible light scratches may be evident as will background noise.  Generally these will not over-power the sound quality but are present.  Sleeves may have seam splits, more significant wear / ring-wear / discolouring

GOOD+/GOOD - Can be played through without skipping. But will likely have significant surface noise, 'ticks' scratches, and visible groove wear. A cover or sleeve will have seam splits, especially at the bottom or on the spine. Tape, writing, ring wear, or other defects will be present.