Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect the materials sector to thrive as our global economy eventually recovers and heats up, fueling construction and manufacturing growth and infrastructure work, the Materials Select Sector SPDR ETF (NYSE: XLB) 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. This materials ETF's expense ratio -- its annual fee -- is an ultra-low 0.20%.

This ETF has performed rather well, beating the S&P 500 over the past three, five, and 10 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 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 contributions to its performance over the past year. Seed-and-herbicide giant Monsanto (NYSE: MON) advanced about 7%. Some investors are worried about the FDA's concern that rootworms may be developing resistance to Monsanto's genetically modified pest-resistant corn seeds, but in response, Monsanto is making lemonade from those lemons by pushing sales of different products.

Other companies didn't add as much to the ETF's returns last year, but could have an effect in the years to come. Cliffs Natural Resources (NYSE: CLF) lost about 15% over the past year, while Freeport-McMoRan Copper & Gold (NYSE: FCX) shed 34%. It's largely our sputtering global economy that's keeping down these stocks and other key commodity companies, such as AK Steel (NYSE: AKS) and U.S. Steel (NYSE: X). Cliffs' management recently said that it expects continued sluggishness in the U.S. economy near term, but that Cliffs itself is in good shape, thanks in part to demand from Asia.

The big picture
Demand for basic materials 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."