Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to focus some of your investing on value stocks that exhibit low book-value-to-price and sales-to-price ratios as well as dividend yields, the Rydex S&P 500 Pure Value ETF (NYSE: RPV) 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 Rydex ETF's expense ratio -- its annual fee -- is a relatively low 0.35%. It's 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 doesn't have a long performance to assess, as it's rather young. It has, on average, topped the S&P 500 handily 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 low turnover rate of 23%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Plenty of value stocks have posted strong performances over the past year. Marathon Petroleum (NYSE: MPC), for example, is up 35% so far this year. Cited as a stock that has it all, it has some investors' expectations high due to its concentration on the promising Gulf and Midwest regions.

Other companies didn't do as well last year, but could rebound in the years to come. Hudson City Bancorp (Nasdaq: HCBK) shed 35%, held back, like many banks, by low interest rates and the prolonged housing market slump. Its recent earnings also took a hit from high mortgage prepayment rates. On the plus side, it's working on improving its profit margins and has improved its credit quality.

Hartford Financial Services (NYSE: HIG) lost 29% over the year, affected by natural disasters, asbestos liabilities, and workers' compensation claims. But it also enjoyed a jump recently, posting expectations-exceeding earnings. Bulls are drawn to its seemingly low valuation and strong expected growth. Some are calling for the company to be split in two, but so far the company is resisting that idea.

Valero Energy (NYSE: VLO) shrank by 7%, but it remains a long-term cash cow, and one that has been turning itself around effectively. It's struggling with weak margins right now, but still holds lots of promise.

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.