This is a basic overview of the differences between the Dow, Nasdaq, and S&P 500.  I knew the basic histories of these three systems, but not the small details.  This article basically sums up in three paragraphs the differences of each market, and how exactly what the market measures differs.  This is a very basic overview, not all that technical by any means, but still some good base knowledge to have.

http://www.freep.com/apps/pbcs.dll/article?AID=/20061018/NEWS99/61018016