Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect companies offering consumer discretionary goods and services to do well as the global economy improves and consumers are able to spend more, the Vanguard Consumer Discretionary ETF (NYSE: VCR) 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 Vanguard ETF's expense ratio -- its annual fee -- is a relatively low 0.19%. (Vanguard is known for very low fees.)

This ETF has performed reasonably well, outperforming the S&P 500, on average, 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 a very low turnover rate of 7%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of consumer discretionary companies had strong performances over the past year. Priceline.com (Nasdaq: PCLN), for instance, surged 45%, again frustrating those who doubt it. The company is experiencing particular success abroad, expanding in Europe and Asia via acquisitions. Those who think its stock is at too high an altitude won't like its recent buyback announcement, especially given that its heady growth rates likely will taper off somewhat soon.

Las Vegas Sands (NYSE: LVS) may have suffered some due to economic challenges in Las Vegas itself, but the company has properties in the lucrative and growing hotspot of Macau (among other locations) and is expanding elsewhere, too, such as in Spain. A recent catalyst for growth is the opening of the new Sands Cotai Central in Macau.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Ford (NYSE: F), for example, sank by 20%. Its sales have been growing, but have not been quite as strong as some had hoped for. Bulls, though, like that its profit margins are likely to grow as it reduces incentives and its focus on fleet sales. It's paying down a lot of debt, too, and its dividend has a good chance of growing.

Meanwhile, Johnson Controls (NYSE: JCI), which makes components and systems for cars and other vehicles, shed 22%. Sales of vehicles may be picking up, but with the economy still sluggish, many people are still putting off purchases as long as possible. In the meantime, the company is investing abroad, with deals in locations such as India and Turkey.

The big picture
Demand for discretionary products may fluctuate, but it's not going away anytime soon. 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.