Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect gold companies to thrive as demand for gold keeps growing in the face of a volatile stock market, the Market Vectors Gold Miners ETF (NYSE: GDX) 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 gold ETF's expense ratio -- its annual fee -- is a relatively low 0.53%.

This ETF has performed well, crushing the S&P 500 over the past five years and especially since late 2008. 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 very low turnover rate of 3%, 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. AuRico Gold (NYSE: AUQ), up 17% so far in 2011, has been firing on all cylinders lately, making major acquisitions and beefing up its production. Well-regarded Newmont Mining (NYSE: NEM), up 12%, offers the intriguing feature of a dividend tied to the spot price of gold. New Gold (AMEX: NGD), also up 12%, has been making some smart deals as it sets itself up for low production costs.

Other companies didn't add as much to the ETF's returns in 2011, but could have an effect in the years to come. Silver Wheaton (NYSE: SLW) may have shrunk by about 14%, but it remains compelling, with analysts boosting their expectations recently. My colleague Chris Barker has even dubbed it "the most profitable company in the world." Hecla Mining (NYSE: HL), down 41%, has tied its dividend to the price of silver -- that will be attractive in times of high or rising prices, but less so at other times.

The big picture
Demand for gold isn't going away anytime soon. 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.