The United States mortgage finance industry

Mortgage lending is a major category of the business of finance in the United States. Mortgages are commercial paper and can be conveyed and assigned freely to other holders. In the U.S., the Federal government created several programs, or government sponsored entities, to foster mortgage lending, construction and encourage home ownership. These programs include the Government National Mortgage Association (known as Ginnie Mae), the Federal National Mortgage Association (known as Fannie Mae) and the Federal Home Loan Mortgage Corporation (known as Freddie Mac). 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, which are known as mortgage-backed securities (MBS).
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, and 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.
The greatly increased rate of lending led (among other factors) to the United States housing bubble of 2000-2006. The growth of lightly regulated derivative instruments based on mortgage-backed securities, such as collateralized debt obligations and credit default swaps, is widely reported as a major causative factor behind the 2007 subprime mortgage financial crisis.