Reddit reviews The Official American Numismatic Association Grading Standards for United States Coins
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The ANA standardized grading based on the Sheldon 70-step scale back in the 1970s (I think). Whitman has published the descriptions in book form for all US coins.
http://coins.about.com/od/Coin-Books-And-Publications/fr/Book-Review-The-Official-ANA-Grading-Standards-For-United-States-Coins.htm
http://www.amazon.com/Official-Standards-American-Numismatic-Association/dp/0307090973
I'm sure if you look around you can find the descriptions online somewhere.
Good dedicated grading guide:
Older version
Newer version
The version shouldn't matter much, though the newer one might have more color photos.