A bond is a debt security, similar to an IOU. Borrowers issue bonds to raise money from investors willing to lend them money for a certain amount of time.
When you buy a bond, you are lending to the issuer, which may be a government, municipality, or corporation. In return, the issuer promises to pay you a specified rate of interest during the life of the bond and to repay the principal, also known as face value or par value of the bond, when it “matures,” or comes due after a set period of time.