Interest is a fee or compensation paid by the borrower to the lender. By way of accumulation, the interest rate can be categorized into simple interest and compound interest. Please check the Compound Interest Calculator to see the difference.
In economics, the interest, or nominal interest rate, includes the current inflation rate and the real interest rate. The average inflation rate in the United States in the past 30 years has hovered around 4.3%. (Please check the Inflation Calculator for more detailed information.) This means you must have an average interest/return rate of 4.3% to not lose the purchasing power of your money. The average annual return rate of the S&P 500 (Standard & Poor's) index in the United States is around 7.5%. TIPS, or Treasury Inflation-Protected Securities, is a bond issued by the U.S. Treasury that provides protection against inflation.
The interest rate of a loan or saving can be "fixed" or "floating". Floating rate loans or savings are normally based on some reference rate, such as the U.S. Federal Reserve (Fed) funds rate or the LIBOR (London Interbank Offered Rate). Normally, the loan rate is a little higher and the savings rate is a little lower than the reference rate. The difference goes to the profit of the bank. Both the Fed rate and LIBOR are short-term inter-bank interest rates, but the Fed rate is the main tool that the Federal Reserve uses to influence the supply of money in the U.S. economy. LIBOR is a commercial rate calculated from prevailing interest rates between highly credit-worthy institutions. The following a diagram of the Fed rate from 1954 to 2009: