The S&P 500 is currently down

.INX 2,773.870 -5.790 -0.208%

About the S&P 500:
The S&P 500 a large cap index for US stocks. The S&P is a weighted average of the 50 largest companies in the United States.

What companies are in the S&P 500?
There are over 500 companies in the S&P 500 (hence the name 500), it covers all leading companies in the US.