Federal Funds Rate - interest rate for overnight loans between banks. While not set directly by the Fed it is largely controled by the Fed through he discount rate, positions the Fed takes in the market. The rate has a great influence on the other rates set by banks and financial institutions for loans. The Federal Reserve Board uses the power to set this rate as a tool to manage the economy. When the board feels the economy is in distress and inflation is controled they may cut the rate. When the board feels the economy may be in danger of overheating and igniting inflation the board may raise the rate.
Historical data from the Federal Reserve web site.