The S&P 500 is arguably the most widely followed index of large-cap American stocks next to the Dow Jones Industrial Average.
As the name suggests, it measures the performance of 500 major firms based in the country, although some have headquarters elsewhere.
Many experts see the health of the S&P 500 as an indication of how well the US economy as a whole is doing and there are a host of investment products linked to its fluctuations.
The firms listed on the index can be traded on either the New York Stock Exchange or the NASDAQ - the two largest trading platforms in the nation.
It represents about two-thirds of the market value of the US stock market and - as a result - can be considered representative of the whole equities industry.
The S&P 500 can be a good benchmark for comparing the performance of stocks, but investors should note that certain economic, political and industrial factors will impact some firms more than others, so such analysis must be kept in context.
During the equities boom of 2007, the index reached its highest ever close of 1,565.15 on October 9th, but this was topped by an intra-day level of 1,576.09 two days later.
Established in 1957, the S&P of the benchmark's title stands for Standard & Poor's, a financial services organisation that operates a number of key indices around the world.
The company's Index Committee decides which firms are included in the reports, basing its decision on the corporations that drive individual sectors, rather than simply picking the largest organisations.
Exxon Mobil, Apple, Chevron, General Electric, Intl Business Machines, Microsoft and JP Morgan are among the top-weighted firms.
A statement from S&P reads: "The index has over $4.83 trillion benchmarked, with index assets comprising approximately $1.1 trillion of this total."
The firm offers several other popular indices, including the S&P 400 and S&P 600, which track the performance of organisations with market capitalisations of between $2 billion and $10 billion, and $300 million and $2 billion respectively.