World of Warcraft, often referred to as WoW, is a massively multiplayer online role-playing game (MMORPG) by Blizzard Entertainment. It is the fourth released game set in the fantasy Warcraft universe, which was first introduced by Warcraft: Orcs & Humans in 1994. World of Warcraft takes place within the Warcraft world of Azeroth, two years after the events at the conclusion of Blizzard's previous Warcraft release, Warcraft III: The Frozen Throne. Blizzard Entertainment announced World of Warcraft on September 2, 2001. The game was released on November 23, 2004, on the 10th anniversary of the Warcraft franchise.
The first expansion set of the game, The Burning Crusade, was released on January 16, 2007. The second expansion set, Wrath of the Lich King, was released on November 13, 2008. The third expansion set, Cataclysm, was announced at BlizzCon 2009.
With more than 11.5 million monthly subscribers, World of Warcraft is currently the world's most-subscribed MMORPG and holds the Guinness World Record for the most popular MMORPG. In April 2008, World of Warcraft was estimated to hold 62 percent of the massively multiplayer online game (MMOG) market.