Special accounts in the Treasury that receive earmarked taxes or other kinds of revenue collections, such as user fees, and from which payments are made for special purposes or to recipients who meet the requirements of the trust funds as established by law.
Of the more than 150 federal government trust funds, several finance major entitlement programs, such as Social Security, Medicare, and retired federal employees' pensions. Others fund infrastructure construction and improvements, such as highways and airports.
Technically, the funds credited to these accounts are restricted by law to their designated programs or uses and are not available for the general purposes of government. Nevertheless, the Treasury borrows from the trust funds for that purpose and the borrowings become part of the public debt.
SOURCE: House Budget Committee
See also: What is the public debt?