Mint (MT) -- A card with four sharp corners, 50/50 centering, no imperfections or signs of wear. Even cards straight out of the pack do not always grade mint.
Near Mint (NM) -- A near-perfect card with perhaps one minor imperfection, such as one blunt corner. Centering is no worse than 60/40 and the card must have its original gloss. NM cards are the standard by which other grades are usually gauged.
Excellent-Mint (EX-MT) -- A card with only two or three minor imperfections. Must have original gloss and centering no worse than 70/30.
Excellent (EX) -- A card with a few minor imperfections. Some original gloss is lost and the edges show moderate wear. All four corners typically show some wear. Centering is no worse than 75/25.
Very Good (VG) -- A card that shows obvious handling but is still attractive despite wear and imperfections. A VG card may have a crease, but one that's not severe enough to make the card unattractive. Most of the original gloss is lost.
Good (G), Fair (F), Poor (P) -- A very worn card with many major imperfections.