“The FOMC decided this week that, with unemployment high and inflation very low, further support to the economy is needed. With short-term interest rates already about as low as they can go, the FOMC agreed to deliver that support by purchasing additional longer-term securities, as it did in 2008 and 2009. The FOMC intends to buy an additional $600 billion of longer-term Treasury securities by mid-2011 and will continue to reinvest repayments of principal on its holdings of securities, as it has been doing since August.” Read more. 

“What the Fed did and why: supporting the recovery and sustaining price stability” 
Ben Bernanke 
Washington Post, November 4, 2010.