Grading coins is difficult and requires special focus and tools to examine coin details. Countries vary in the grading system they use. For example the U.S. and Canada have a slightly different system. As well, certification services offered by companies can also differ in the grade given to the same coin. 

A "grade" measures a coin's appearance. There are generally five main components which determine a coin's grade: strike, surface preservation, luster, coloration and eye appeal. Grading is subjective and even experts can disagree about the grade of a given coin.

To learn more about grading, the American Numismatic Association (ANA) has a grading game to try at this link: 

NGC has a guide to coin grading at this link. (click picture above)

PCGS has grading standards at this link. (Click picture above.) 

Heritage Coins has an on-line grading tutorial at this link. (Click picture above.) 

CoinsCanada offers an easy to follow view of grading for Canadian coins at this link. (Click picture above.)