From Diablo Wiki
Electronic Sports or E-sport refers to the sport played by professional gamers, their tournaments and leagues.
While e-sports and pro-gaming have grown quite popular in some parts of the world, the developers of Diablo III are not focusing on that aspect of gaming. Jay Wilson explained  that spending time and effort on the e-sports aspect would take away from the work they could do on other features they feel are more important to the quality and success of Diablo III. There will be support for competitive play and PvP duelling in D3, but it won't be a major part of the game, as it is in RTS and FPS games.
Pro-gaming is most common in Korea, where StarCraft became a popular pastime about the same time as they had economical problems, and the IT sector was growing. Hundreds of thousands of Koreans played (and play) StarCraft and other games in PC baangs (gaming cafes). Even though most gamers have a computer at home, many prefer to play in baangs with a public social atmosphere.
With games becoming spectator sports in the baangs, some matches were televised, and when they drew respectable ratings, televising the contests became a common event in Korea. The profession has grown over time until the top pro gamers these days have coaches, practice special exercises to boost their gaming skills, and can earn a good living from prize money and endorsements.
The most popular games have active fan communities, with the top "pro-gamers" enjoying a level of celebrity comparable to professional athletes.
- Team Liquid's Article on Progaming (Team Liquid is a famous pro-gaming team)