There are two rates set by the Federal Reserve: the fed funds rate and the discount rate. For definitions see below.
The fed funds rate is more important because all short-term rates are derived from the fed funds rate. If the Federal Reserve cuts the fed funds rate, all short-term rates are similarly cut, which has a major impact on the economy.
The discount rate, on the surface, does not have such a major impact, since few loans are done at the discount rate, see the definition of discount rate below. The Federal Reserve recently (8/17/2007) cut just the discount rate, and the impact being that it would soon cut the fed funds rate (perhaps on 9/18/2007).
Few organizations can move the market like the Federal Reserve. As an investor, it's important to understand exactly what the Fed does and how it influences the economy.
For more information on The Federal Reserve see http://www.investopedia.com/university/thefed/
Understand the various factors that influence them so you can learn to anticipate their movements for profit. (Trying To Predict Interest Rates - http://www.investopedia.com/articles/03/122203.asp
Learn about the tools the Fed uses to influence interest rates and general economic conditions, see Formulating Monetary Policy - http://www.investopedia.com/articles/04/050504.asp
More information can be found by submitting "definition of fed funds rate" to answers.com, including a graph of historical fed funds rates and additional links.
The fed funds rate is interest rate at which a depository institution lends immediately available funds (balances at the Federal Reserve) to another depository institution overnight. In fact, the Federal Reserve sets a target for this rate, but not the actual rate itself (because it is determined by the open market).
The discount rate is the rate at which the Fed lends directly to banks.