An index of 30 blue-chip US stocks, the DJIA at more than 100 years old is the oldest ongoing US market index and the most watched number in the world.
From time to time, old stocks are rotated out of the DJIA and new ones are put in to "better represent" the current economic situation. In the latest such change (late 1999), Microsoft and Intel were two of the four stocks placed in this very watched index.
more information is available here: http://indexes.dowjones.com/faqs.html