It seems pretty obvious to me that the three major indexes followed in the US are the Dow Jones Industrial Average, NASDAQ and the S&P 500. There are literally hundreds of different indexes listed in the US (http://www.bloomberg.com/markets/indexes/country/usa/). Why are these three the particular ones that are commonly followed and thus have various index funds and ETFs which track them? And furthermore, there are dozens of indexes that being with the words "S&P 500". How do these "S&P 500" indexes relate to the S&P 500?