Special drawing rights are a supplementary foreign exchange reserve asset defined and maintained by the International Monetary Fund. SDRs are the units of account for the IMF, and not a currency; rather, SDR value is based on a basket of currencies; the currency choice is reviewed every five years. SDRs represent a claim to currency held by IMF member countries for which they may be exchanged. When SDRs were created in 1969, they were each worth 0.888671 grams of gold, roughly the equivalent of one US dollar at the time. In 1974, the IMF redefined the SDR as equivalent to the value of a specific selection of world currencies.
SDRs are allocated by the IMF to IMF member countries and central banks, and cannot be held or used by private entities or individuals. The number of SDRs in existence was around XDR 21.4 billion in August 2009. During the 2008 financial crisis, an additional XDR 182.6 billion was allocated to "provide liquidity to the global economic system and supplement member countries' official reserves". By October 2014, the number of SDRs in existence was XDR 204 billion.Wikipedia