“The SDR is an international reserve asset, created by the IMF in 1969 to supplement its member countries’ official reserves. Its value is based on a basket of four key international currencies, and SDRs can be exchanged for freely usable currencies. With a general SDR allocation that took effect on August 28 and a special allocation on September 9, 2009, the amount of SDRs increased from SDR 21.4 billion to SDR 204 billion (equivalent to about $308 billion, converted using the rate of August 31, 2010).”
Here is the official factsheet of SDRs at the IMF’s website.