S&P 500

The S&P 500 follows 500 of the biggest US companies, weighted by their market value. When people say "the market was up today," they very often mean this index. It is maintained by S&P Dow Jones Indices, and the companies are picked by a committee using size, profitability and other criteria.

For forex traders it is the main thermometer of risk sentiment. A falling S&P often goes hand in hand with a stronger yen and Swiss franc, and a rising one with stronger risk currencies like the Aussie dollar.