You probably know a little bit about the S&P 500, right? It's a collection of 500 of America's biggest companies, whose performance is tracked collectively through the S&P 500 index. Together, they represent more than 75% of the value of the entire U.S. stock market, which includes several thousand stocks. If you invest in an S&P 500 index fund, it will be a mutual fund invested in the same 500 companies, in the same proportions, giving you the same performance (roughly, once fees are deducted). So far, so good.

Did you know, though, that there are lots of different indexes, and that many of them are represented by various index funds? Whatever your investing goal is, there's a decent chance that an index exists that can help you invest. Want small caps? Go for the Russell 2000. Want the entire U.S. stock market, including both large and small caps? Check out the Dow Jones Wilshire 5000.

In today's volatile market, many people are looking to traditionally "defensive" stocks, which tend to remain relatively steady in market downturns. The folks at Sabrient Systems have recently introduced a new twist on that idea, with their new Sabrient Defensive Equity Index. This index includes 100 companies, and some are the typical "defensive" sorts, such as pharmaceutical, energy, and food companies, which sell things people need in any economy. For example, the fund's recent holdings included PepsiCo (NYSE:PEP), Merck (NYSE:MRK), Consolidated Edison (NYSE:ED), and Kellogg (NYSE:K).

However, the index also includes companies from other industries that have solid balance sheets, reliable dividends, conservative accounting practices, and a strong track record during market downswings. These include Barrick Gold (NYSE:ABX), Lockheed Martin (NYSE:LMT), and Whirlpool (NYSE:WHR). You can invest in this index through the Claymore/Sabrient Defender ETF (DEF).

What to do
I wouldn't rush to invest in this exchange-traded fund, but it might suit your needs. Its expense ratio (the annual fee, that is) is 0.79%, although it has an expense cap of 0.60%. That's rather steep compared with typical index funds, but it's on the low side when compared with managed mutual funds. Some S&P 500 index funds carry expense ratios of less than 0.10%, while managed stock funds tend to have ratios above 1.00%. 

The ETF offers a yield of around 1.8%, which is not unattractive, but the fund remains unproven, without even a three-year track record to examine. Its turnover, at 21% per year, is low compared with most managed mutual funds, but it's somewhat high for an index fund. And the higher the turnover, the more the fund will rack up in trading fees and taxes. So go ahead and consider this fund, but don't buy in until you've done more research.

Find your own
Another approach to investing that suits today's market, as well as pretty much any other market, is to employ some superior stock-picking skills and seek out healthy, growing, undervalued companies. Learn more about how to invest when everything is falling.

If you'd like some guidance and leads in your quest for strong companies, test-drive our Motley Fool Inside Value newsletter service. A free trial will give you access to all past issues and all recommended stocks.

Longtime Fool contributor Selena Maranjian owns shares of PepsiCo. Try our investing newsletter services free for 30 days. The Motley Fool is Fools writing for Fools.