If you are familiar at all with US bonds, what is a CUSIP is something you will be familiar with. They are unique numbers used for bonds and in security trading. The most important use given them is in processing and settling bonds. It is the 9 character number used in the bond market. Trading companies number into the thousands in the United States alone.
Most bonds issued are municipal ones that are rendered to states, counties, or cities and villages. Numbering in excess of one million, these bonds need a unique numbering system to keep track of them all. It is the Treasury Department that is the issuer of these bonds, so the first six numbers of a CUSIP identifies that department. This will be followed by two letters that will specifically identify the kind of bond that was issued. The final two digits have no relation to the bond but is a means to ensure that the bond has not been tampered with.