Country of Origin
Country of origin is one of the most basic of all coin classification systems. All of a country's various coins fall under this category, regardless of their other characteristics. On most coins, the country of origin is written on one of the coin faces. United States coins would be an example of this classification, as would Mexican coins or Chinese coins.
Denomination
The denomination of a coin is simply how much it is worth. All coins worth the same amount are categorized together, regardless of style or condition. An example of this is the fact that all 10-cent U.S. coins are classified as dimes. United States coins use the denominations of penny, nickel, dime, quarter, half-dollar and dollar. Historically, other denominations have been used such as 2-cent piece and 20-cent piece.
Style
The style of a coin refers to the images engraved on the faces of coins. Generally this classification goes along with denomination. For example, wheat pennies and Lincoln Memorial pennies are both different styles of pennies. Most coin styles are descriptively named or named after their designer (like the famed St. Gaudens $20 gold coins).
Year
Coins can also be classified as to their year of production. The year of mintage is almost always present on one of the faces of a coin. All coins minted in a specific year would fall under this classification regardless of denomination or country of origin. In this way, a 2003 Canadian nickel can fall under the same classification umbrella as a 2003 Washington Head quarter.
Physical Condition
The physical condition of a coin is one of the most important factors in determining the value of a coin. Coins are physically graded on a scale from poor to proof (special coins made specially for collectors). The better the condition, the more a coin will be worth. All uncirculated coins can be classified together by virtue of their condition. The range of grades includes poor, fair, good, fine, extra fine, uncirculated and proof. Smaller divisions can be made to more accurately grade coins.