A bonds is a debt security in which the authorized issuer owes the holders a debt and depending on the terms of the bonds is obliged to pay interest (the coupon) and/or to repay the principal at a later date, termed maturity. (It is a formal contract to repay borrowed money with an interest at a fixed intervals).When you purchase a bonds; you are lending money to the issuer which may be a government, corporation, federal agency, or other entity. In return for the loan, the issuer promises to pay you a specified rate of interest during the life of the bonds and to pay back the face value of the bonds or the principal when it “matures,” or comes due.