A coin collector is sometimes called a "numismatist," though numismatics is the study or collection of currency, including paper money. In general use, the term "coin collector" is more common.
Coin collecting can be a casual hobby or a form of investment. As with other collectibles, prices fluctuate according to supply and demand. Older coins tend to be worth more, as do coins with misprints or other errors. Specimens that are in exceptionally good condition are more valuable, particularly uncirculated coins. Collectors sometimes clean older coins in the hope of bringing a better sale price, but cleaning reduces the value of an old coin. Professional coin dealers are adept at spotting a cleaned or polished coin.