The Dow Jones Industrial Average is a price-weighted average of 30 significant stocks traded on the New York Stock Exchange and the Nasdaq. The DJIA was invented by Charles Dow back in 1896.
Often referred to as "the Dow", the DJIA is the oldest and single most watched index in the world. The DJIA includes companies like General Electric, Disney, Exxon and Microsoft.
When the TV networks say "the market is up today", they are generally referring to the Dow.