The S 500 is arguably the most followed stock index in the world. It's existed in its current form since 1957 and tracks the 500 largest and most profitable publicly traded companies in the United States. Consequently, it's a benchmark for the stock market and the economy.

The obvious implication is that the constituents of this index are some of the biggest and the most influential companies in the world. If they're succeeding or struggling overall, chances are other companies are too.

Here's the thing, though: A list of profitable, important, and publicly traded companies isn't static -- things are constantly changing. And the S 500 is constantly updating which companies make the cut. In just the past year, Airbnb, Uber Technologies, and Super Micro Computer were added to the S&P 500 index.

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Source Fool.com