The full story of the nature and origin of the financial crisis specifically in the subprime mortgage market is told elsewhere, but here is a sketch of the background. The U.S. government created, at different times, two organizations to provide a secondary market for home mortgages. A secondary market is where primary lenders such as banks can sell the mortgage they have created. The government-created agencies for providing a secondary market in home mortgages are known as Fannie Mae and Freddie Mac. Aroung 1960 they became private companies. Fannie Mae was probably the largest business in the world in terms of asset holdings.
In the 1990's Fannie Mae announced that it would not deal with lenders who red-lined; i.e., refused to lend to buyers who were wanting to purchase homes in poor and perhaps dangerous neighborhoods. The lenders agreed to those terms, but Fannie Mae told them that they would have to prove that they were not red-lining by issuing quotas of mortgages to ethnic minority buyers. When the lenders could not meet those quotas and still maintain their standards for lending Fannie Mae and Freddie Mac encouraged the lenders to lower those standards and they would insure or buy these subprime mortgages.
The subprime mortgages were not good investments. They were made worse by charging the subprime borrowers a higher interest rate which just increased the risk of default. For the lenders it did not matter how poor of an investment the subprime mortgages were as long as they could sell them immediately in the secondary mortgage market. The lenders got a fee for writing the mortgage and the size of the fee was substantially larger for the subprime mortgages.
Meanwhile Fannie Mae and Freddie Mac began to bundle mortgages, prime and subprime, and sell securities based upon the mortgage payments deriving from the pool of mortgages. They even created securities with a gradation of risks ranging from those with first claim to the mortgage payments to those with last claim, called toxic waste. Fannie Mae, Freddie Mac and others creating mortgage-backed securities appeared to be making a profit on this process of securitization. But the subprime mortgages were actually a loss for Fannie Mae and Freddie Mac. They paid too much for the subprime mortgages and not enough for the default insurance they were providing. It only took ten years for the social policy imposed in the late 1990's to bankrupt the largest business in the world.
As the market for subprime mortgages and the market for securities based upon those mortgages grew there was increased an increased market for default insurance. This took the form of credit default swaps (CDS's). The CDS's were a form of derivative security.
The market for derivative securities has become very large in recent years. Worldwide these securities provided in the 1990's "insurance" on an estimated $16 trillion of financial securities. This is an enormous amount, far larger than the gross domestic product (GDP) of the United States at the time. However it pales in comparison to the level reached in the 2000's. According to the International Swaps and Derivatives Association (ISDA) the notional value of the CDS's in 2007 worldwide in 2007 was $62.2 trillion. The total GDP of all the countries in the world in 2008 was about $60 trillion. The total value of household real estate in the U.S. at the time was only about $19.9 trillion. This indicates that much of the activity was not in providing insurance against risk but was sheer speculation.