Three of the key actors in the tragic play that was the economic meltdown are the major credit rating agencies: Moody's, Standard & Poors, and Fitch. First, we should establish that these organizations, despite being referred to as "agencies", are not part of the government.
To appreciate the important role they play in the financial ecosystem, we must introduce a few other actors. Among the largest investors are "institutions". These organizations have enormous sums of money to invest. A prime example is a state's pension fund, an organization that manages a pile of money (which may be $100 billion or more) to pay the state's employees when they retire.