Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect emerging markets to grow over the coming years, and to need new infrastructure, the PowerShares Emerging Markets Infrastructure ETF (NYSE: PXR) 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 PowerShares ETF's expense ratio -- its annual fee -- is 0.75%, which is a bit higher than the typical ETF, but also considerably lower than the average stock mutual fund.

This ETF has performed reasonably well, beating the S&P over the past three years, but it's still very young. 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 turnover rate of 36%, 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 reasonable contributions to its performance over the past year. Caterpillar (NYSE: CAT), up 6%, joins many of its peers in expanding its operations in areas such as Brazil. It recently agreed to provide generator sets for supply vessels off the coast of Brazil. Manitowoc (NYSE: MTW) is another major infrastructure specialist profiting from Brazil, where it's building a huge crane plant.

Other companies didn't add as much to the ETF's returns last year, but could have an effect in the years to come. Brazilian steel company Gerdau (NYSE: GGB) shrunk by about 41%, but as Brazil builds for the 2016 Olympics and the 2014 World Cup, among many other projects, it's likely to enjoy increasing demand. Vale (Nasdaq: VALE), off by 30%, is also suffering from insufficient demand for steel -- but remember that steel is a cyclical business, and downturns don't last forever.

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.

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.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.