Dow Jones Industrial Index

The Dow Jones Industrial Average is a widely followed barometer of stock performance. It, and its sibling the Down Jones Transportation Index, are believed to be the oldest indexes of stock performance in the world.

Summary From Wikipedia
The Dow Jones Industrial Average (DJI), also called the DJIA, Dow 30, or informally the Dow Jones or The Dow) is one of several stock market index created by nineteenth century Wall Street Journal editor and Dow Jones & Company co-founder Charles Dow. Dow compiled the index as a way to gauge the performance of the industrial component of America's stock markets. It is the oldest continuing U.S. market index, aside from the Dow Jones Transportation Average, which Dow also created.

"The Dow" Today
Today, the average consists of 30 of the largest and most widely held public companies in the United States. The "industrial" portion of the name is largely historical&mdash;many of the 30 modern components have little to do with heavy industry. The average is price-weighted. To compensate for the effects of stock splits and other adjustments, it is currently a scaled average, not the actual average of the prices of its component stocks&mdash;the sum of the component prices is divided by a divisor, which changes whenever one of the component stocks has a stock split or stock dividend, to generate the value of the index. Since the divisor is currently less than one, the value of the index is higher than the sum of the component prices.