The U.S. dollar is currently the dominant reserve currency in the world. There are other currencies considered reserve currencies, e.g. the Euro, Japanese Yen and Pound Sterling.
Crude oil is priced in U.S. Dollars. Other commodities are priced in U.S. dollars. Countries of the world buy and sell to each other and the currency used for exchange is U.S. dollars. The following article published on the International Monetary Fund website discusses the U.S. dollar as reserve currency. The Dollar Reigns Supreme, by Default, FINANCE & DEVELOPMENT, March 2014, Vol. 51, No. 1, Eswar Prasad, author. http://www.imf.org/external/pubs/ft/fandd/2014/03/prasad.htm
Using a reserve currency allows world commerce to flow easier. Many countries hold financial instruments in reserve currencies because these currencies are more stable and reliable than other currencies. Two countries doing import/export business with each other can “pay” each other in a reserve currency to finalize their transactions.
Below are two (2) more reference sites.
U.S. Treasury website
Appendix 1: An historical perspective on the reserve currency status of the U.S. Dollar