Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect companies in emerging markets to thrive as their populations grow and move up into the middle class, the iShares MSCI Emerging Markets Index ETF (NYSE: EEM) 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 lots 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 -- comes in at 0.69%.

This ETF has performed rather well, topping the broader index of developed international companies over the past three and five years, on average, though it's underperforming so far this year. 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.

With a low turnover rate of 14%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Several of this ETF's components made strong contributions to its performance over the past year. Brazil-based beer distributor AmBev (NYSE: ABV), a subsidiary of Anheuser-Busch InBev (NYSE: BUD), gained about 30% over the past year, operating mostly in the growing markets of Latin America. Brazil is the world's third-largest beer market.

Other companies didn't add as much to the ETF's returns last year but could have an effect in the years to come. Latin America's largest private bank, Itau Unibanco (Nasdaq: ITUB), shed about 33% over the past year but is growing rapidly and expanding, recently buying HSBC's (NYSE: HBC) Chilean retail division. Its CEO expects profits to rise in the near term, as interest-rate cuts decrease loan delinquencies.

The big picture
Emerging markets are likely to keep emerging for many years. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Learn about the best dividend ETFs. And if you're looking for some great investments beyond ETFs, consider these 10 Stocks for Your Retirement Portfolio.

Longtime Fool contributor Selena Maranjian holds no position in any company mentioned. Check out her holdings and a short bio. 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.