As of December of 2014, the United States owes more than $18 trillion. This debt includes both debt held by the public and intragovernmental holdings.
Debt held by the public includes all federal debt that is held by individual people, corporations, and state, local and foreign governments. It also includes debt held by corporations, Federal Reserve banks and entities other than the U.S. government. Debts held by the public include notes, bonds, U.S. Treasury bills, U.S. savings bonds, TIPS and government series securities.
Intragovernmental holdings are securities that are held in trust funds, special funds and revolving funds by the government. These holdings also include securities of the Federal Financing Bank, or FFB. The FFB is a government corporation that was created by Congress and is supervised by the Secretary of the Treasury. Its purpose is to reduce the cost of borrowing for the federal government.
To put the vastness of the public debt into perspective, the debt increased a total of $794 billion in 2013. This averages out to an increase of around $90.6 million per hour, or around $1.51 million per minute. If the total wealth of the 25 richest people was added together, it would amount to only $722 billion, which is less than the increase in the federal debt in just one year.