Investors in the U.S. stock market typically follow major indexes: the S&P 500, Nasdaq Composite, and Dow Jones Industrial Average. While all three are important in their own right, the S&P 500 has the largest amount of assets tracking its performance. By including 500 of the largest public U.S. companies by market cap, the S&P 500 has become the benchmark for the U.S. stock market. When people talk about the stock market's performance broadly, they're often referring to the S&P 500.

The S&P 500's importance isn't unwarranted, either. Containing industry leaders from every major sector, it's one of the best gauges of how the overall economy is doing. As the overall U.S. economy goes, so goes the S&P 500 -- which is often a good thing. If you're wondering if there's one investment that can help you retire a millionaire, look no further.

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Source Fool.com