Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect emerging markets to thrive over time as their economies grow faster than ours, the SPDR S&P Emerging Markets ETF (NYSE: GMM) 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 SPDR ETF's expense ratio -- its annual fee -- is a relatively low 0.59%. That's a bit higher than many ETFs, but also considerably lower than the typical stock mutual fund. The ETF is rather 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 rather well, outperforming its benchmark over the past three and five 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.

With an ultra-low turnover rate of 4%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of companies based in emerging markets had strong performances over the past year. Taiwan Semiconductor (NYSE: TSM), for example, gained 26%; it makes a range of chips for other companies and dominates its peers by market share. The company is bullish on its future, and plans to invest nearly $12 billion in new production capacity over the coming years. Some expect it to soon begin supplying more chips for Apple, which could boost its top line considerably.

China-based search engine giant Baidu (Nasdaq: BIDU) gained just 4% over the past year, but its future looks promising. Despite China's slowing economic growth rate, the company is still expected to increase its revenue and earnings by more than 40% in 2012 and 2013. It, too, could benefit from partnership with Apple if, as is rumored, its search engine is parked on Chinese consumers' iPhones.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Brazil-based oil and gas giant Petroleo Brasileiro (NYSE: PBR), down 36%, has been adding to its reserves and boosting production aggressively. It has discovered a lot of oil recently, and is viewed as a solid value by some.

Brazil-based metals specialist Vale (Nasdaq: VALE), which also produces fertilizers, shed 28% partly due to concerns over China's slowdown, and partly due to labor issues , tax concerns, and lawsuits . Despite all that, it stands to benefit from an uptick in demand as the global economy rebounds; while investors wait, the stock offers a dividend yield north of 5%.

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 4 ETFs You Can Count On. And if you're looking for some great investments beyond ETFs, consider these 5 Stocks Growing Their Dividends by 20% Per Year.