Mortgage lending is a major category of the business of finance in the United States of America. Mortgages are commercial paper and can be conveyed and assigned freely to other holders. In the U.S., the Federal Housing Administration administers the programs colloquially known as "Ginnie Mae", Fannie Mae and "Freddie Mac" (also known as the GSEs or government sponsored entities ) to foster mortgage lending and thus to encourage home ownership and construction. These programs work by buying a large number of mortgages from banks and issuing (at a slightly lower interest rate) "mortgage-backed bonds" to investors known as MBS or Mortgage Backed Securities.

This allows the banks to quickly relend the money to other borrowers (including in the form of mortgages) and thereby to create more mortgages than the banks could with the amount they have on deposit. This in turn allows the public to use these mortgages to purchase homes, something the government wishes to encourage. The investors, meanwhile, gain low-risk income at a higher interest rate (essentially the mortgage rate, minus the cuts of the bank and GSE) than they could gain from most other bonds.

Securitization is a momentous change in the way that mortgage bond markets function which has grown rapidly in the last 10 years as a result of the wider dissemination of technology in the mortgage lending world. For borrowers with superior credit, government loans and ideal profiles, this securitization keeps rates almost artificially low, since the pools of funds used to create new loans can be refreshed more quickly than in years past, allowing for more rapid outflow of capital from investors to borrowers without as many personal business ties as the past.

Source: http://en.wikipedia.org/wiki/Mortgages