Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you want to add a bunch of companies experiencing significant growth to your portfolio, the Vanguard Growth ETF (NYSE: VUG) 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 Vanguard ETF's expense ratio -- its annual fee -- is an ultra-low 0.12%. (Vanguard is known for very low fees.)

This ETF has performed rather well, beating the S&P 500, on average, over the past three- and five-year periods. 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 23%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of growth companies had strong performances over the past year. Apple (Nasdaq: AAPL) is perhaps the best example, rising 77% as it became the most valuable company on the U.S. stock market by market capitalization. Bulls drool, imagining huge sales of Apple TV devices, lower-cost iPhones, and new iterations of iPads and other items. Bears wonder whether open-source systems will overtake Apple's closed-system offerings, and worry about legal wranglings.

Philip Morris International (NYSE: PM) advanced about 40%, with investors optimistic about its future as economies around the world develop, creating many more middle-class people who can afford to smoke. It also doesn't hurt that many foreign countries don't tax and regulate tobacco as much as the U.S. does. Its free cash flow has been growing briskly, but so has its debt. My colleague Nick Kapur recently cited Philip Morris as one of five stocks to own for 50-plus years.

Other companies didn't do as well last year but could see their fortunes change in years to come. Boeing (NYSE: BA) shed 4%. It finally brought its Dreamliner 787 to market, but Air India is demanding compensation for the delays -- making some wonder if other airlines will follow, and whether Boeing will set a precedent by paying up. Boeing also won't be reaping huge profits on these planes for a while.

Meanwhile, Caterpillar (NYSE: CAT) lost about 2% -- though it has average annual gains of 18% over the past 20 years. The company's business is very cyclical, meaning that it stands to benefit as the global economy recovers and construction projects proliferate. During the recent down cycle, the company has been investing internationally, including in Asia.

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 5 ETFs That Could Soar in 2012. And if you're looking for some great investments beyond ETFs, consider these 12 Dividend Stocks for 2012.

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.