The federal government issues public and intragovernmental debt, with both marketable and non marketable classifications.
The most common debt held by the public is in the form of marketable treasuries (bills, notes, bond) with smaller amounts held in government account series (GAS) which are earmarked for specific purposes, savings bonds (series EE, etc.), and state and local government series (SLGS) which are held by municipalities.
Intragovernmental debt is money that the government owes itself, which is generally held in various trust funds. The most notable are funds for Social Security, Medicare, and the Military Retirement fund. These are generally designed to be self-funding accounts and are not usually included when analyzing the overall size of the federal debt. They are, however, considered when analyzing the total assets and liabilities.