Bonds expire or come due on their maturity date, at which time the entire value of the bond is repaid. Bonds can be bought and sold like any other investment. While their rate of return is not as high as stocks, they also don't have the same level of risk. One of the reasons for this is that a bondholder gets paid first before stockholders, if a company goes bankrupt. Risk becomes even less a factor when you buy bonds through mutual funds.
Many experts recommend bond mutual funds due to the less liquid nature of bonds over stocks.
In most cases, the higher the expected return on the bond, the higher the risk of losing principal. Generally, though, the longer the maturity of the bond, the more volatile the price. Short-term bonds are considered the safest option. But, bond investment isn't always safe. Companies, cities and counties occasionally do go bankrupt and those loans may not be paid back.