Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect the oil services industry to thrive over time as our reliance on oil doesn't disappear anytime soon, the Market Vectors Oil Services ETF (AMEX: OIH) 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.

If you're familiar with ETFs and think that the OIH ticker looks familiar, but the fund's name doesn't, you're right. This ETF is one of several replacing Merrill Lynch-based HOLDRs which had some problems that these funds don't.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The Market Vectors ETF's expense ratio -- its annual fee -- is a low 0.35%.

This ETF doesn't have much of a performance record yet, as it's just a few weeks old. It's relatively 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 oil services stocks have done well over the past year. Seadrill (NYSE: SDRL), up 20%, has been dubbed "the best offshore drilling stock" by my colleague Travis Hoium. It's very focused on ultra-deep drilling, has been benefiting from the high price of oil, and sports a tantalizing dividend yield above 8%. National Oilwell Varco (NYSE: NOV), up 9%, has a hefty backlog of orders, and is well-positioned to serve busy shale and deepwater drilling operations.

Other companies didn't do as well last year, but could have an effect in the years to come. McDermott International (NYSE: MDR) shed 36%, but it also recently inked a big new contract, its biggest ever, at $2 billion. Pushing its stock down over the past year was a slump in its Middle East operations and some difficulties in shifting into deeper drilling.

Transocean (NYSE: RIG), the world's largest offshore drilling contractor, fell 38% over the year. Some bulls see it as an attractive acquisition target and like its hundreds of millions of dollars in backlog orders, and the fact that it has been raising some of its prices lately. Bears worry about drilling disasters, such as a recent one off the coast of Russia.

The big picture
Demand for oil is likely to be with us for quite a while. 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 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.