The Fed

In simplest terms, the Fed is the central bank responsible for the stability of our money, the US dollar.  History has shown that the creation of money cannot be trusted to politicians -- perhaps the first modern example is the 1720 collapse of John Law's bank established by royal edict in France ("The Mississippi Scheme," described in "Extraordinary Popular Delusions and the Madness of Crowds," by Charles Mackay.)  An independent central bank is necessary, but not easy.

The Fed publishes an online book about its purpose and function here.