Gem Mint: Absolutely PERFECT both in physical condition and in centering.  Even many new cards pulled straight from packs are not truly Gem Mint.

Mint: Perfect.  No excuses, no defects, no rounded corners, no nicks, nothing.  Slightly off center might be acceptable depending on the card.  I personally feel that off center isn't as bad a flaw as actual damage since the card was made off center and the owner(s) had nothing to do with that flaw.  Some cards, like the Kellogg's 3-D cards will have a slight curvature, even on Mint specimens.  It takes a really spectacular card for me to grade it Mint.

Near Mint: A near perfect card, but it will have no more than 1 or 2 tiny, minor flaws.

Excellent: A few tiny flaws or wear spots. Visually, Excellent cards look great but don't stand up to close scrutiny.

Very Good / EX: A very nice card that has maybe 4 or 5 small flaws that keep it from being graded an Excellent.  This is one of those grades that can change depending on how you are feeling that day (Hey - at least I'm honest!).  No creases. 

Very Good+: Slight corner wear on most corners with maybe the very lightest of surface creases

Very Good: Light corner wear all over and maybe a light crease or two but nothing really bad.  It can be tough to find pre-1970 cards in better than VG, if they are graded properly.

Good: Worn, but complete.  Rounded corners, creases, thumbtack holes, scuffs, pretty much what cards degrade to after being in your pocket or bicycle spokes.

Less than Good: Very worn and might be missing a corner or has been used for BB gun practice.  Use these as fillers until a nicer specimen comes along or put them in the spokes of your bicycle for some retro fun.