Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect small-cap companies to thrive in coming years, the iShares S&P Small Cap 600 Growth (NYSE: IJT) ETF could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in several dozen of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The iShares ETF's expense ratio -- its annual fee -- is a low 0.25%.

This ETF has performed well, outperforming the S&P 500 over the past three, five, and 10 years. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

What's in it?
Several of this ETF's components made strong contributions to its performance over the past year. TriQuint Semiconductor (Nasdaq: TQNT) surged 64%, selling components to communications equipment makers. Its chips have been in Apple (Nasdaq: AAPL) iPads and iPhones, with a quarter of its revenue last year coming from Apple contractor FoxConn. Darling International (NYSE: DAR) is in a vastly different business, rendering animal carcasses and now expanding into biofuels. Its stock has been volatile but advanced 146% over the past year.

Other companies didn't add quite as much to the ETF's returns last year but could have an effect in the years to come. Stifel Financial (NYSE: SF) rose 23%, for example, while Coinstar (Nasdaq: CSTR) climbed 24%. Many have been predicting the death of Coinstar, with its RedBox DVD rental kiosks competing against mighty Netflix (Nasdaq: NFLX), but the company recently reported robust results and is adding video-game rentals to its mix. (Netflix, meanwhile, is rapidly growing its streaming distribution and is expanding internationally as well.) Stifel has been enjoying rapid revenue growth, but it's now undergoing an SEC investigation into its collateralized debt obligation sales.

The big picture
A well-chosen ETF can grant you instant diversification across an industry or market segment -- and can make investing in and profiting from it that much easier.

ETFs can help you find the way to better investing results. To find some great ETF investing ideas, take a look at The Motley Fool's special free report, " 3 ETFs Set to Soar During the Recovery ."

Longtime Fool contributor Selena Maranjian owns shares of Netflix and Apple, but she holds no other position in any company mentioned. Check out her holdings and a short bio. The Motley Fool owns shares of TriQuint Semiconductor and Apple. Motley Fool newsletter services have recommended buying shares of Netflix, Coinstar, and Apple, creating a bull call spread position in Apple, and buying puts in Netflix. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.