The SDR is an international reserve asset, created by the IMF in 1969 to supplement its member countries’ official reserves. As of March 2016, 204.1 billion SDRs (equivalent to about $285 billion) had been created and allocated to members. SDRs can be exchanged for freely usable currencies.
The value of the SDR is based on a basket of five major currencies—the US dollar, the euro, the Chinese renminbi (RMB), the Japanese yen, and the British pound sterling.” (from the IMF website)
For a comprehensive review on the current role of the SDR, the basket of currencies that determines its value and its interest rate, the SDR allocations to IMF members and buying and selling transactions in SDRs please see the IMF’s Factsheet.