NEAR MINT - A nearly perfect copy that looks brand new with only a few very minor defects. Acceptable minor defects on a NM copy include: A very small amount of spine stresses without color breaks, very minor instances of denting (two or three at most), slight corner blunting, and minor (less than 1/8") bends without color breaks.
VERY FINE - A VF copy has minor defects, but is in overall excellent condition. Most well-kept modern comics (especially if they have been read) fall into this grade. Acceptable defects on a VF are minor and include: Minor corner wear, a light accumulation of spine stress that may include color-break, a light accumulation of dents, and bends or folds less than 1/4" (note that on a VF copy, some color-break is allowed in a bend/fold).
FINE - A comic in FN condition is considered "above average" but still displays some wear. In general, the eye appeal is somewhat reduced due to either an accumulation of minor defects or one or two moderate defects. Acceptable defects on a FN copy include: Slight spine roll, a moderate accumulation of spine stresses that may break color, a spine split of less than 1/2", minor water spotting or residue (less than the size of a dime), an impacted corner, and moderate foxing.
VERY GOOD - A comic in VG condition shows some significant wear, but has not accumulated enough total defects to reduce eye appeal to the point that it is not a desirable copy. A VG copy may have an accumulation of minor defects or one or two major ones. Acceptable defects on a VG copy include: Spine roll, 1/2" to 1" spine splits or other tears, a cover or centerfold that is detached at one staple, discoloration due to oxidation, and a moderate accumulation of water damage or staining.
GOOD - A GD copy has major defects, but is still complete and readable. A GD copy will have a significant amount of damage, usually an accumulation of smaller defects punctuated with some major defects. Acceptable defects on a GD copy include: A vertical book-length crease, 1.5"-2" spine split, cover or centerfold completely detached, major tears, heavy discoloration/brittleness due to oxidation, heavy amounts of staining, residue, and water damage.
FAIR - A FR is the lowest grade a comic book can receive as long as the story and art are complete. A FR copy will have virtually no eye appeal and will display major damage. A comic book in FR condition may have non-story elements such as coupons, ad pages, or Marvel Value Stamps cut or torn out of the book. Types of damage that place a comic in FR range include: A spine split of up to 2/3 the length of the book, a missing back cover provided the front cover is still attached, severe water damage or residue damage, mold, and paper deterioration due to oxidation.