What is securitization?
First, a regulated and authorized financial institution originates numerous mortgages, which are secured by claims against the various properties the mortgagors purchase. Then, all of the individual mortgages are bundled together into a mortgage pool, which is held in trust as the collateral for an MBS. The MBS can be issued by a third-party financial company, such as a large investment banking firm, or by the same bank that originated the mortgages in the first place. Mortgage-backed securities are also issued by aggregators such as Fannie Mae or Freddie Mac.
Regardless, the result is the same: a new security is created, backed up by the claims against the mortgagors' assets. This security can be sold to participants in the secondary mortgage market. This market is extremely large, providing a significant amount of liquidity to the group of mortgages, which otherwise would have been quite illiquid on their own. (For a one-stop shop on subprime mortgages, the secondary market and the subprime meltdown, check out the Subprime Mortgages Feature.)
Furthermore, at the time the MBS is being created, the issuer will often choose to break the mortgage pool into a number of different parts, referred to as tranches. These tranches can be structured in virtually any way that the issuer sees fit, allowing the issuer to tailor a single MBS for a variety of risk tolerance profiles. Pension funds will typically invest in high-credit rated mortgage-backed securities, while hedge funds will seek higher returns by investing in those with low credit ratings.