There are many types of coin collection themes. Many collectors focus on coins and paper money from certain countries. Others focus on certain years, specific mints, or historical periods.

In the U.S., some of the most popular coins include gold and silver content coins. The Morgan Silver Dollar (minted from 1878 to 1921) are very popular among collectors due to their rarity and relative affordability.

Early dollar coins, dated from 1794 to 1804, are also highly sought after. Gold coins like the St. Gaudens Double Eagle are extremely desired because: 1. Gold coins were melted down during wartime; 2. They were designed by famous sculptor Augustus St. Gaudens.

Popular collectible paper money from the U.S. includes Federal Reserve issue notes which are extremely hard to find in good condition. Colonial notes and shillings, as they are often referred to, were printed by independent presses and usually on thicker paper than contemporary currency.

Currency from the Civil War, one of the most studied wars in U.S. history, is also extremely popular among collectors. Not only do coin collectors search for currency from this period, Civil War historians and experts desire it too. 3-cent trimes are an example of Civil War time currency that is becoming harder and harder to locate.

To see a list of various types of collections, see this Wikipedia page.