Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect the global materials industry to thrive as the world economies recover and building and growing activities heat up, the iShares S&P Global Materials ETF (NYSE: MXI) 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 materials ETF's expense ratio -- its annual fee -- is a relatively low 0.48%.

This ETF has performed reasonably well, beating the S&P 500 over the past five years, though slightly underperforming it over the past three. 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 4%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Several essential materials companies had strong performances over the past year. Monsanto (NYSE: MON), for example, gained 16%; investors are excited about its forays into emerging markets and its expansion into soybean and wheat seeds, in addition to its big business in corn. There's some uncertainty around Monsanto's future, though, as more than 2,000 farmers and others are urging the EPA and USDA to look into the risks presented by new weed killers being developed by Monsanto and other companies. That has hurt the stock's performance in the past few months.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. DuPont (NYSE: DD), for instance, shed 1%. It has seen strong contributions from its agriculture business; in its latest quarter, the agricultural division generated about 36% of revenue, and grew by 16% over the prior year.

Fertilizer giant PotashCorp (NYSE: POT) shrank by 21%, but there are signs that demand may grow robustly for its offerings, and the three-company cartel to which it belongs recently signed a contract with China.

Freeport McMoRan Copper & Gold (NYSE: FCX) gave up 27%, recently posting poor results, partly due to labor problems in Indonesia. There are plenty of reasons to be bullish on the company, though, such as expectations that copper prices will grow due to demand from the automobile industry and infrastructure projects, and the company's vital molybdenum business.

The big picture
Demand for essential materials isn't going away anytime soon -- which is why they're dubbed "essential." 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 four ETFs you can count on. And if you're looking for some great investments beyond ETFs, consider these five stocks growing their dividends by 20% per year.