From the Summary: "Total debt of the federal government can increase in two ways. First, debt increases when the government sells debt to the public to finance budget deficits and acquire the financial resources needed to meet its obligations. This increases 'debt held by the public.' Second, debt increases when the federal government issues debt to certain government accounts, such as the Social Security, Medicare, and Transportation trust funds, in exchange for their reported surpluses. This increases 'debt held by government accounts.' The sum of 'debt held by the public' and 'debt held by government accounts' is the total federal debt. Surpluses reduce debt held by the public, while deficits raise it. Total federal debt outstanding was $14,345 billion on May 31, 2011. The U.S. Treasury Secretary Timothy Geithner announced that the federal debt reached its statutory limit on May 16, 2011, and that he had declared a debt issuance suspension period, allowing certain extraordinary measures to extend Treasury's borrowing capacity until early August 2011. […] The recent economic slowdown led to sharply higher deficits in recent years, which led to a series of debt limit increases."
CRS Report for Congress, RL31967