Here is a long piece that does a great job of clearly explaining what the heck is going on in the financial markets.

This episode started when the Treasury nationalized Fannie Mae and Freddie Mac on September 8. Their combined assets are over $5 trillion. These firms help guarantee most of the mortgages in the United States. The Treasury only got authority from Congress to take this action in July, and in seeking the authority had insisted that no intervention would be needed.

The Treasury has replaced the management of both companies and will presumably oversee their operation. This decision marked an acknowledgment by the government that the mortgage market and the institutions to make it operate in the U.S. are broken.

On Monday, the largest bankruptcy filing in U.S. history was made by Lehman Brothers. Lehman had over $600 billion in assets and 25,000 employees. (The largest previous filing was WorldCom, whose assets just prior to bankruptcy were just over $100 billion.)

On Tuesday, the Federal Reserve made a bridge loan to A.I.G., the largest insurance company in the world…

The trigger in each case was the inability to get financing, although the reasons differed in each case.

The Fannie and Freddie situation was a result of their unique roles in the economy. They had been set up to support the housing market. They helped guarantee mortgages (provided they met certain standards), and were able to fund these guarantees by issuing their own debt, which was in turn tacitly backed by the government. The government guarantees allowed Fannie and Freddie to take on far more debt than a normal company. In principle, they were also supposed to use the government guarantee to reduce the mortgage cost to the homeowners, but the Fed and others have argued that this hardly occurred. Instead, they appear to have used the funding advantage to rack up huge profits and squeeze the private sector out of the ” conforming” mortgage market. Regardless, many firms and foreign governments considered the debt of Fannie and Freddie as a substitute for U.S. Treasury securities and snapped it up eagerly.

Fannie and Freddie were weakly supervised and strayed from the core mission. They began using their subsidized financing to buy mortgage-backed securities which were backed by pools of mortgages that did not meet their usual standards. Over the last year, it became clear that their thin capital was not enough to cover the losses on these subprime mortgages. The massive amount of diffusely held debt would have caused collapses everywhere if it was defaulted upon; so the Treasury announced that it would explicitly guarantee the debt.

But once the debt was guaranteed to be secure (and the government would wipe out shareholders if it carried through with the guarantee), no self-interested investor was willing to supply more equity to help buffer the losses. Hence, the Treasury ended up taking them over.

The concern for the man on Main Street is not the bankruptcy of Lehman, per se. Rather, it is the collective inability of major financial institutions to find funding.

As their own funding dries up, the remaining financial firms will be much more cautious in extending credit to normal firms and individuals. So even for people whose own circumstances have not much changed, the cost of the credit is going to rise. For an individual or business that falls behind on payments or needs an increase in short-term credit because of the slowing economy, credit will be much harder to obtain than in recent years.

This is going to slow growth. We have not seen this much stress in the financial system since the Great Depression, so we do not have any recent history to rely upon in quantifying the magnitude of the slowdown. A recent educated guess by Jan Hatzius of Goldman Sachs suggests that G.D.P. growth will be just about 2 percentage points lower in 2008 and 2009. But as he explains, extrapolations of this sort are highly uncertain.

[All emphasis is mine.]