Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect oil and gas equipment stocks to prosper over time because of our planet's persistent dependence on oil, the iShares Dow Jones US Oil Equipment Index ETF (NYSE: IEZ) 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 -- is a relatively low 0.47%.

This ETF has a bit of a mixed performance record, beating the world market over the past three years, roughly matching it over the past five, and badly underperforming it over the past 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 13%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Some oil and gas equipment companies had strong performances over the past year. Chart Industries (Nasdaq: GTLS), for example, specializing in equipment used in the production of hydrocarbon and industrial gas, advanced 19%. Bulls who expect natural gas to be fueling many vehicles in the near future like the company, as it supplies storage tanks, among other things. The company clearly believes in the future of liquefied natural gas, as it's been beefing up its LNG infrastructure manufacturing capacity to meet expected demand. Still, if natural gas prices rise considerably, the stock's future may not look quite as bright.

Many other companies in the industry didn't do as well last year, but could see their fortunes change in the coming years. Weatherford International (NYSE: WFT), for example, sank 44%, partly due to an estimate that it will fork over some $100 million in settlements with multiple U.S. agencies investigating possible improper practices abroad. Bears worry that there may be further bad news and costs. The company has been plagued with tax-related problems, as well. On the plus side, though, Weatherford recently reported solid revenue growth, though its earnings disappointed analysts a bit.

Offshore drilling specialist McDermott International (NYSE: MDR) shed 41%, but sports a hefty and growing backlog of orders. Two months ago the stock surged 10% on news that the CEO and three other bigwigs bought sizable bunches of stock. There aren't many explanations for that other than the executives expecting the company's stock to rise.

Then there's Halliburton (NYSE: HAL), down 37%. Yes, it has reported flat earnings recently, but they still exceeded analyst expectations, and revenue also beat expectations, rising 22%. The company has been very involved in fracking, but some see a possible pullback as natural-gas producers have been reining in their operations. Halliburton's operational diversification as well as its geographic reach bode well for the company, though, and many see it as undervalued these days.

The big picture
Demand for energy 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.

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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 Halliburton. The Motley Fool has a disclosure policy.

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