T-notes are medium-term loans to the U.S. government. They come in terms of 2-years, 3-years, 5-years, 7-years, and 10-years. They can be purchased directly from the U.S. government or from a bank which purchases them for you and can be purchased in units of $100.
T-bills have maturities of one year or less, while T-bonds have maturities of 20 or 30 years.
These bonds pay a coupon, that is a semi-annual interest payment. They are considered very safe investments and are actively traded in the active market. Because of this, the purchase price may be above or below the par value or face price of the T-note (usually $100) . However, when the note matures, you are paid the face value of the note.
The notes can be purchased via a non-competitive bid, which means you will receive the amount of notes you want, but the yield is outside your control and is determined by auction. They can also be purchased via a competitive bid, which means you may or may not receive all the notes you want at the yield you want, depending on what the yield is determined by auction. 
Most financial news reports mention something along the lines of "bonds were up today, which brought the yield down to ..." (The price and yield move in opposite directions.)
T-notes are backed by the credit of the U.S. government, and thus are considered as close to a risk-free investment. The yield for the appropriate term Treasury note (or bill or bond) is used in the capital asset pricing model as part of the determination of the cost of capital or required rate of return.
The current and recent historical yields can be obtained from the U.S. Treasury at http://www.ustreas.gov/offices/domestic-finance/debt-management/interest-rate/yield.shtml.
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