Special drawing rights or SDRs are a form of currency created by the International Monetary Fund and its member countries to supplement the Fund's currency reserves. The value of SDRs is based on a basket of major traded currencies - the dollar, the euro, sterling and the yen - and fluctuates as the currencies change. SDRs serve as the unit of account of the IMF and a number of other international organizations. SDRs are not a useable currency nor are they a direct claim on the IMF.

See also: http://www.imf.org/external/np/exr/facts/sdr.htm