Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you're drawn toward stocks capable of aggressive growth but don't want to choose just a few of them, the Russell Aggressive Growth ETF (AMEX: AGRG) 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 Russell ETF's expense ratio -- its annual fee -- is a low 0.37%.

This ETF doesn't have much of a performance record yet, as it's still less than a year old. It's very small, too, so if you're thinking of buying, beware of occasionally large spreads between its bid and ask prices. You might want to just keep an eye on it as it matures a bit, or you might want to be an early investor. Remember that as with most investments, 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?
Several stocks in the index that this ETF tracks performed well in the past year. Qualcomm (Nasdaq: QCOM) gained 12%, as investors waxed bullish over the proprietary technologies that it licenses to others and the chips it sells, many of which are housed in iPhones. (Bears worry that its profit margins might slip due to falling royalty rates and selling prices.) Monsanto (NYSE: MON) advanced 16%, posting strong sales results recently and growing its market share. It's also growing internationally, with its pesticide penetration still very low in many markets.

Caterpillar (NYSE: CAT), meanwhile, rose 11%, expanding into fast-growing international markets such as China and India and recording very strong revenue growth. Its promising $8.6 billion acquisition of mining equipment maker Bucyrus was also a factor in its stock's rise over the past year.

Other companies didn't perform as well but could bounce back in the years to come. United Parcel Service (NYSE: UPS), for instance, gained just 7%, but its prospects look good, especially as the U.S. Postal Service flounders. It's also growing internationally and in some sectors, such as health-care transportation services.

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.

Longtime Fool contributor Selena Maranjian owns shares of Qualcomm, but she holds no other position in any company mentioned. Click here to see her holdings and a short bio. The Motley Fool owns shares of United Parcel Service and Qualcomm. Motley Fool newsletter services have recommended creating a synthetic long position in Monsanto. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.