1) The rate that banks can borrow money from the Federal Reserve.
2) The rate used to discount future cash flows to determine the current value of the instrument.
3) The rate used in certain money market instruments e.g. U.S. Treasury Bills, when there is no coupon. The interest is said to be paid up front i.e. the interest amount is calculated first using the discount rate and the appropriate conventions and then subtracted from the principal. Therefore the equivalent yield will always be higher than the discount rate.