Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add growing small-cap stocks to your portfolio because of their ability to grow rapidly, the iShares S&P Small Cap 600 Growth ETF (NYSE: IJT) 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 small-cap ETF's expense ratio -- its annual fee -- is a relatively low 0.25%.

This ETF has a strong performance record, 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.

What's in it?
Plenty of small-cap companies had strong performances over the past year. Questcor Pharmaceuticals (Nasdaq: QCOR), which specializes in drugs for central nervous system and inflammatory disorders, more than doubled -- it advanced 118%. Its multiple sclerosis treatment Acthar has been selling well; the drug is also effective against kidney disease, so Questcor is pursuing that market as well. Revenue and earnings both have been growing at an average rate of more than 30% annually over the past three years.

Cirrus Logic (Nasdaq: CRUS) surged 68%. With more than 1,000 patents serving more than 700 products, it's a compelling proposition. It doesn't hurt that it's supplying audio chips to Apple, among other customers. The company is also producing a power management module for LED lighting systems and has an order backlog for it.

Satellite and wireless specialist ViaSat (Nasdaq: VSAT) gained 20%, offering a fast new satellite broadband service called Exede that targets rural customers and looks like it will successfully compete with DSL and cable. ViaSat's revenue and earnings both grew at 16% or more over the past year.

Coinstar (Nasdaq: CSTR), up 18%, may be known for its coin machines in supermarkets, but it's also the company behind those ubiquitous Redbox DVD kiosks. The company is looking to offer streaming video services, too, via a partnership with Verizon. That could put pressure on Netflix, but Coinstar will face a lot of tough competition.

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. And if you'd like to profit off the trillion-dollar mobile revolution with stocks other than Cirrus Logic, check out this free special report to learn about the surprising star of the mobile revolution.

Longtime Fool contributor Selena Maranjian, whom you can follow on Twitter, owns shares of Netflix and Verizon Communications, 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 Apple and Cirrus Logic. Motley Fool newsletter services have recommended buying shares of Coinstar, Apple, and Netflix, as well as a bull call spread position on Apple. The Motley Fool has a disclosure policy.

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