A mortgage-backed security (MBS) is a type of asset-backed security which is secured by a mortgage or collection of mortgages. The mortgages are aggregated and sold to a group of individuals that securitizes, or packages, the loans together into a security that investors can buy. Bonds securitizing mortgages are usually treated as a separate class, termed residential; another class is commercial, depending on whether the underlying asset is mortgages owned by borrowers or assets for commercial purposes ranging from office space to multi-dwelling buildings.

In finance, a bond is a type of security under which the issuer (debtor) owes the holder (creditor) a debt, and is obliged – depending on the terms – to provide cash flow to the creditor. The timing and the amount of cash flow provided varies, depending on the economic value that is emphasized upon, thus giving rise to different types of bonds. The interest is usually payable at fixed intervals: semiannual, annual, and less often at other periods. Thus, a bond is a form of loan or IOU. Bonds provide the borrower with external funds to finance long-term investments or, in the case of government bonds, to finance current expenditure.