Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect companies based in emerging markets to thrive over time, as their home economies and many economies they serve are growing more rapidly than economies such as ours, 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 a lot 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 0.67%, which is a bit higher than many ETFs, but also considerably lower than the typical stock mutual fund.

This ETF has performed well, surpassing the overall world market over the past three and five 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.

With a low turnover rate of 17%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of emerging-market companies had solid performances over the past year. America Movil (NYSE: AMX) gained 14%; it belongs to Carlos Slim, who has overtaken Warren Buffett and Bill Gates as the world's richest man. The company has seen its earnings slip recently (partly due to exchange rates), but its wireless subscribership is growing, and has topped 250 million. It has long focused on Latin and North America, but is growing in Europe via some recent acquisitions. In the U.S., its pay-as-you-go TracFone service is growing, too, with more than 21 million subscribers.

South Africa-based oil and chemical company Sasol (NYSE: SSL) advanced 5%. Already experienced in converting coal and gas to liquid, it has been converting natural gas to liquid in a commercially viable way. Synthetic fuels generate about half the company's operating profits.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Brazil-based Vale (NYSE: VALE), the world's second-largest mining company, shed 27%, but recently sported eight out of 10 characteristics of the perfect stock. It has been whacked by slowing growth in China, but for patient investors it offers a dividend yield near 6%. It's also Brazil's largest railroad operator and is looking to bid for rights to many more miles.

Petrobras (NYSE: PTR), Brazil's top oil company, is down some 18% over the year. Some have been worried about the company's expansion plans not moving along quickly enough. On the plus side, though, Petrobras recently announced a major discovery in Brazil's offshore "subsalt" area -- a 400-meter oil column. The company plans to spend about half of its exploration funds in the subsalt region over the next few years.

The big picture
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.

You can add a lot of international exposure to your portfolio without leaving home, too. Check out our free report, "3 American Companies Set to Dominate the World." These American companies are taking advantage of high-growth emerging markets. Click here to get your free copy before it's gone.

Longtime Fool contributor Selena Maranjian, whom you can follow on Twitter, holds no position in any company mentioned. Click here to see her holdings and a short bio. Motley Fool newsletter services have recommended buying shares of Petrobras and Sasol. The Motley Fool has a disclosure policy.

We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.