An interest rate is a rate that is charged on a loan or a rate given for a deposit that is a percent of the total money amount. Excess interest rates are called usury and are against the law. Financial institutions can usually charge interest rates that are greater than individuals can charge when they make private loans to other individuals.
Interest rates can be either fixed (fixed rates) or they can change (adjustable rates). Rates on savings and checking accounts in a bank will usually change over time whereas a certificate of deposit will usually have a fixed rate for the life of the contract.