Treasury Bills (T-bills) are the most marketable money market security. Essentially, T-bills are a way for the U.S. government to raise money from the public. They are exempt from state and local taxes. Other positives are that T-bills (and all Treasuries) are considered to be the least risky investment in the world because the U.S. government backs them.
T-bills are short-term securities that mature in one year or less from their issue date. They are issued with three-month, six-month and one-year maturities. T-bills are purchased for a price that is less than their par (face) value; when they mature, the government pays the holder the full par value. Effectively, your interest is the difference between the purchase price of the security and what you get at maturity.