Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some emerging markets stocks to your portfolio but don't have the time or expertise to hand-pick a few, the SPDR S&P Emerging Markets ETF (NYSEMKT: 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 lots 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 0.59%, and it yiedls about 2.5%. The fund is fairly small, too, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This ETF has underperformed its benchmark over the past three and five years, but the future matters more than the past. With many stocks in this realm having slid, some now see them as more attractively priced. 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.

Why emerging markets?
Emerging markets rightfully draw the attention of many investors because they can offer greater growth rates as their economies develop.

More than a handful of emerging-markets companies had strong performances over the past year. Chinese search-engine giant Baidu (BIDU -2.02%), for example, advanced 19%. It has suffered as China's economic growth has slowed and has also faced tough competition. Its second quarter featured strong earnings and a rosy outlook. Bulls like its profitability and growth prospects, such as in video and smart TVs. Its shares also got a boost recently on news that it's buying a major Chinese mobile apps company. The company has been frustrating its short-sellers with its surging price.

Other companies didn't do quite as well over the last year, but could see their fortunes change in the coming years. Brazil-based Vale (VALE 1.07%), the world's largest iron-ore concern, gained just 2% and sports a 5.2% dividend yield. With a forward P/E near 7, about half of its five-year average of 14, the stock looks undervalued to many. Some are not convinced, though, sensing continued pressure on commodity prices. Its second quarter offered estimate-topping earnings, but revenue below expectations, with management expecting improvement because of production increases and effective cost-cutting. Conditions in Brazil haven't helped, with protesters calling for improved infrastructure and services instead of massive spending on upcoming Olympics and World Cup games.

China Life Insurance (LFC) shed 5%. In its first quarter, it reported operating earnings up 79%, leading to an analyst upgrade to Outperform from Zacks. Zacks sees China's economy improving and upped its projections for China Life. China Life itself has raised its own forecast, expecting first-half earnings to jump by some 50% over year-ago levels. The company is one of the world's largest life insurers, but doesn't seem to be near bargain levels lately.

Brazil's PetroBras (PBR 1.50%) sank 34%. It has been burdened by significant debt and challenged by government interference in its business. Bulls have been heartened by rising production numbers as some offshore rigs are brought back into service and some are hopeful that solid car sales in Brazil will boostPetrobras' business, too. It has a $237 billion investment program under way, and expects to double its production in coming years. Analysts at Zacks have downgraded it to underperform, though, in part because of erratic earnings.

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.