A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
The S&P 500 is an index of the shares of the 500 largest publicly traded companies in the United States and a key indicator of its economic health. It is a market-value weighted index which is also float adjusted. This means that the market capitalization used to weight each share in the index is calculated using the number of shares available for public trading not the total issued. For most companies in the S&P 500 their market capitalization is the same as their float adjusted value.
See also: http://www2.standardandpoors.com/portal/site/sp/en/us/page.topic/indices_500/2,3,2,2,0,0,0,0,0,0,0,0,0,0,0,0.html
FTSE 100, FTSE Eurotop100 and 300 Indices, Nikkei 225, DJIA, NASDAQ