Bond ETFs are portfolios of bonds that track the performance of a bond or index, such as 20-year Treasuries. They are bought and sold like stocks. Investors reap interest via monthly dividends, and any capital gains are paid out through an annual dividend. Bond ETFs don't mature, but they reflect the particular bond index's maturity and can be shorted, traded on margin and hedged with options. They offer investors many advantages: low expense ratios, close tracking to benchmarks, flexibility and ease of use, stop and /or limit orders, a round lot is not necessary, there are no investment minimums and options are available on many ETFs.
Joanne Von Alroth discusses some other advantages to using bond ETFs. It costs less to buy a bond ETF than to buy varying bonds of different maturities -- known as laddering. There's historical transparency in pricing and you avoid fees on each individual bond trade and the spreads that are built into stocks.
Also, you spread your risk if you invest in an ETF that tracks the 100 bonds on the corporate bond index -- such as iShares Lehman Aggregate Bond Fund (AGG) -- rather than putting all your money behind one or two corporate bonds.
Here is a great comparison of iShares' fourteen fixed income ETFs.