The Standard and Poor’s 500 index is made up of the top 500 companies in the US, where “top” refers to market capitalization. As a side note, market cap refers to the total dollar amount of the company’s outstanding shares and is used by the investment community to determine a company’s size. The S&P 500 represents about 80% of the total stock market’s value and is, therefore, a good indicator for the US markets as a whole.