An umbrella term: The government sells various Treasuries to investors. Buying a Treasury amounts to lending money to the government in return for some profit.
The term "Treasuries" usually refers to US Treasury Bills or US Treasury Notes. They are obligations of the US government sold to finance the federal deficit. Interest on these bonds is taxable on your federal income tax return but free of state and local income taxes.
The interest rates paid on these bonds is closely monitored. The yield curve that results from plotting the interest rates out to 30 years is the standard to which all other fixed income investments are referenced. New issue corporate bonds are often priced based on their differential from the Treasury yield curve. The differential is usually a function of the bond rating. Hence, bond rating firms express market interest rates for fixed income investments by reporting the average differential from the yield curve for bonds of a given bond rating.
The shape of the yield curve is an economic indicator, and is much discussed in the financial news. An inverted yield curve is an indicator of a pending recession. (It frequently indicates the Federal Reserve Board is pushing up short term interest rates--usually in an effort to slow an overheated economy.)