The International Organization for Standardization (ISO) created a currency standard to develop codes for use in trade and banking. The organization agreed to a three-letter alphabetic code that is determined by the country abbreviation and the country's corresponding currency. The standard is updated as needed.
For instance, the United States' currency is the dollar; therefore, the code is USD. Japan's currency is yen; therefore, the code is JPY. The complete list of countries and their currency codes are available at Wikipedia.org or for a small fee at iso.org.