Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you're a fan of investing guru Louis Navellier and are interested in investing in the 100 stocks his quantitative system deems to be of the highest quality in the U.S. market, the RevenueShares Navellier Overall A-100 ETF (NYSE: RWV) 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. In fact, the ETF aims to outperform the index.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The RevenueShares ETF's expense ratio -- its annual fee -- is 0.60%. That's higher than that of many ETFs, but also considerably lower than that of most stock mutual funds. (The fund is also very tiny, so if you're thinking of buying, beware of large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.)

This ETF is too young to have a sufficient track record to assess. It outperformed the S&P 500 in 2010, and underperformed it in 2011. 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 steep turnover rate of 190%, this fund, like many of its mutual fund peers, isn't committing to its picks for very long. Low turnover rates can reflect more conviction.

What's in it?
Several Navellier-picked companies had strong performances over the past year. Melco Crown (Nasdaq: MPEL) gained 92%, benefiting from its concentration on Macau, where casino companies have been thriving. Many think it sports a compelling valuation, but some worry about shrinking market share.

Snowmobile and all-terrain vehicle maker Polaris Industries (NYSE: PII) gained 72%, despite a warm winter in many parts of the country. It recently reported estimate-beating revenue and earnings, and hiked its dividend by a whopping 64%. Its ATV business, not surprisingly, has seen more demand recently.

Tractor Supply (Nasdaq: TSCO), up 65%, has been a long-term winner for investors, averaging more than 30% in annualized gains over the past decade. The retailer of farming equipment and more sports over 1,000 stores in some 44 states and believes there's room for some 800 more. Its profit margins have been steadily growing in recent years.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Green Mountain Coffee Roasters (Nasdaq: GMCR) has been frustrating naysayers for many years now, repeatedly surpassing expectations. It hit a bump recently, though, and is down some 12% over the past year. That's partly due to looming patent expirations and also to Starbucks announcing its own home-brewing single-serve coffeemaker. Still, many continue to view the shares favorably.

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.