Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you want to add a bunch of foreign stocks to your portfolio but don't have the time or expertise to hand-pick a few, the RevenueShares ADR ETF (NYSE: RTR) 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.

Unlike many international funds, this ETF weights its holdings by revenue, not market capitalization. That results in some smaller companies having more influence on it. It also doesn't base itself on the MSCI EAFE index, which focuses mostly on Europe, the Far East, and Asia. It casts a wider net.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The RevenueShares ETF's expense ratio -- its annual fee -- is a relatively low 0.49%. The ETF itself is 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.

This ETF has performed reasonably, but it's also very young, with just a few years on the books. On average, it outperformed the S&P 500 over the past three years, but 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 reasonable turnover rate of 37%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of foreign-based companies had strong performances over the past year. PetroChina (NYSE: PTR), for example, gained 16%. Investors are bullish on its future due to China's growing population, which will drive energy demand up. PetroChina has been busy investing in oil and gas fields around the world lately. Fellow oil giant BP (NYSE: BP), up 3%, has many eager investors waiting for it to put the Gulf oil spill behind it by settling or winning litigation cases. Some investors have been dismayed by the company's behavior in its legal proceedings, though.

Other companies didn't do as well last year but could see their fortunes change in the coming years. Steel company ArcelorMittal (NYSE: MT), down 39%, has suffered in our global economic malaise. But as the recovery continues, its prospects will improve, and low energy prices can help even more. In the meantime, its diversification and valuation are appealing.

Telefonica (NYSE: TEF), Europe's largest telecom company, sank 25%, as it suffers along with much of Europe. It's likely to prosper over the long term, but the near term and mid-term may offer more of the same: pressure on revenue and profits as Europe digs its way out of trouble.

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.