Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to invest in most of the overall market in one fell swoop, but would also like to boost your exposure to smaller and undervalued companies, the FlexShares Morningstar U.S. Market Factor Tilt Index ETF (NYSE: TILT) could save you a lot of trouble. It invests in more than 2,500 companies and boosts its holdings of small-caps and value stocks. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of promising ones simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The Tilt ETF's expense ratio -- its annual fee -- is a relatively low 0.27%. The fund is rather small, too, so if you're thinking of buying, beware of occasionally large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This ETF doesn't have much of a track record yet since it's so new. 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.

What's in it?
Plenty of companies had strong performances over the past year. Apple (Nasdaq: AAPL), for example, surged about 58%, and that's including a recent decline. Bulls see it as still a bargain, with a P/E ratio under 15 and gobs of growth ahead. Bears worry that it can't keep hitting home runs with every innovation, and fret about the new upcoming Windows operating system and wireless carriers frustrated with high iPhone fees.

Philip Morris International (NYSE: PM), meanwhile, gained 25%, offering a solid 3.7% dividend yield and a history of dividend growth. Smoking may be waning in the U.S. due to increased taxes and regulations, but it's growing in other parts of the world, where the population is also growing briskly. The company is buying smokeless-nicotine technology, too, to help it develop new product offerings, and it recently posted estimate-topping revenue and earnings results, along with rising profit margins.

Other companies didn't do as well last year, but they could see their fortunes change in the coming years. Schlumberger (NYSE: SLB) shrank about 22%, but it's also an R&D leader in its field, offering high-tech services to the oil and gas industry, which will likely keep needing them for a long time. It has upped its revenue and earnings by double-digits recently, and growth seems to be accelerating. Schlumberger is also diversified geographically, with operations in plenty of fast-growing regions, such as Latin America.

General Electric (NYSE: GE) was basically flat over the past year, but it would be silly to bet against its future. Bears can find reasons to worry, but the company is spread across many lucrative industries, and has been investing heavily in new technologies, such as alternative power. After cutting its dividend in the recent fiscal crisis, GE has been raising it significantly, and its GE Capital division is actually showing signs of strength.

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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.

Longtime Fool contributor Selena Maranjian, whom you can follow on Twitter, owns shares of Apple, but she holds no other position in any company mentioned. Click here to see her holdings and a short bio. The Fool owns shares of Apple. Motley Fool newsletter services have recommended buying shares of and creating a bull call spread position in Apple. The Motley Fool has a disclosure policy.

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