Mt- = Near Mint: In brand new condition with no surface marks or deterioration in sound quality. Cover and contents nearly perfect.
Ex = Excellent: Shows some  signs of being played but there is very little lessening in sound  quality. Cover and contents might have slight wear and/or creasing.
VG+ = Better than Very Good: Obviously  been played but no significant deterioration in sound quality despite  light surface marks. Very little wear and tear on cover and contents,  without any major defects.
VG = Very Good: Obviously been played many times but no major deterioration in sound quality despite  noticeable surface marks and occasional light scratch. Normal wear and  tear on cover or contents, without any major defects.
Gd+ = Better than Good: Sound quality  has deteriorated but with no distortion. Light scratches and occasional  mild scratch. Cover and contents suffer from folding, scuffing of edges  but not spine splits or discoloration.
Gd = Good: Sound quality has  noticably deteriorated, perhaps with some distortion. Mild scratches.  Cover and contents suffer from folding, scuffing of edges, spine splits, discoloration etc.
Fair: Still just playable but  has not been cared for properly and displays considerable surface noise - it may even jump. Cover and contents will be torn, stained and/or  defaced.
Poor: Will not play properly due to scratches, bad surface noise etc. Cover and contents will be badly damaged or partly missing.