When the US Government doesn't have enough money raised through tax revenues to cover the spending, it sells Treasuries to make up for the deficit. These debt obligations are interest bearing.
The issuer is the US Department of Treasury.
There are three types:
- Bills - Maturity under 1 years
- Notes - Maturity between 1 and 10 years
- Bonds - Maturity 10 - 30 years
The rate on Treasuries is what are used to form the yield curve.