Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you like mid-cap companies because they've proven themselves enough to grow to a significant size and because they still have a lot of room for growth, the Focus Morningstar Mid Cap ETF (NYSE: FMM) 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 Morningstar ETF's expense ratio -- its annual fee -- is a very low 0.12%.

This ETF doesn't have much of a performance record yet, as it's less than a year old. It's relatively 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. 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 16%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Several mid-cap stocks posted strong performance over the past year. Hard-drive specialist Seagate Technology (Nasdaq: STX) surged 87%, partly on price hikes due to suppliers being hurt by flooding in Thailand. That situation also led Seagate to change its distribution system, from using shipping hubs to shipping directly from factories. As a vote of confidence in itself, the company also hiked its dividend by 39% recently.

Up 23%, Health Care REIT (NYSE: HCN) is attracting investors with its focus on properties such as residential care facilities and doctors' offices, which should enjoy growing demand as our population ages.

Other companies didn't perform as well last year, but could have an effect in the years to come. Weyerhaeuser (NYSE: WY) and Fifth Third Bancorp (Nasdaq: FITB) each shed 12% over the past year. Weyerhaeuser owns some 20 million acres of timberland. It will benefit from the eventual rebound of the housing market, and may also benefit from inflation. As a new real-estate investment trust (REIT), though, it may also suffer some when interest rates eventually begin rising, as REITs today are especially valued for the income they can provide. Fifth Third has been busy improving its credit quality, reducing both its commercial and consumer non-performing assets.

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.