In short: The S&P measures the 500 largest companies in the U.S.A
The S&P 500, or simply the S&P, is a stock market index that measures the stock performance of 500 large companies listed on stock exchanges in the United States. It is one of the most commonly followed equity indices, and many consider it to be one of the best representations of the U.S. stock market. The average annual total return of the index, including dividends, since inception in 1926 has been 9.8%; however, there were several years where the index declined over 30%. The index has posted annual increases 70% of the time.
For a list of the components of the index, see list of S&P 500 companies. The components that have increased their dividends in 25 consecutive years are known as the S&P 500 Dividend Aristocrats.
The S&P 500 is a capitalization-weighted index and the performance of the 10 largest companies in the index account for 21.8% of the performance of the index.
Funds that track the index have been recommended as investments by Warren Buffett, Burton Malkiel and John C. Bogle for investors with long time horizons.
Although the index includes only companies listed in the United States, companies not originating from the United States might be included in the future. It already includes many multi-national companies; companies in the index derive on average only 71% of their revenue in the United States.
The index is one of the factors in computation of the Conference Board Leading Economic Index, used to forecast the direction of the economy.
The index is associated with many ticker symbols, including: ^GSPC, INX, and $SPX, depending on market or website. The index value is updated every 15 seconds, or 1,559 times per trading day, with price updates disseminated by Reuters.
The S&P 500 is maintained by S&P Dow Jones Indices, a joint venture majority-owned by S&P Global and its components are selected by a committee.
From: Wikipedia “The S&P500”