When governments need money, beyond that which they raise in taxes, they borrow money by issuing bonds, which banks, institutions or sometimes individuals buy.

Bonds normally offer a yield, so instead of holding cash, the bond pays guaranteed interest for its term, currently around  3.5% to 4.5% per annum.

The shortest period for a bond is around 4 weeks, the longest has just been issued by Google to UK pension funds and lasts for 100 years, yes 100 years.

At the end of the period, you get your money returned to you. In the case of government bonds that returned money is produced by issuing more bonds.

Bonds are known by many other terms: 

Money printing
Quantitative Easing 
Inflation 

are the most common terms.