World of Warcraft, often referred to as WoW, is a massively multiplayer online role-playing game (MMORPG). It is Blizzard Entertainment's 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 world of Azeroth, four years after the events at the conclusion of Blizzard's previous release, Warcraft III: The Frozen Throne. Blizzard Entertainment announced World of Warcraft on September 2, 2001. The game was released on November 23, 2004, celebrating 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.
With more than 11.5 million monthly subscribers, World of Warcraft is currently the world's largest MMORPG in those terms, and holds the Guinness World Record for the most popular MMORPG. In April 2008, World of Warcraft was estimated to hold 62% of the massively multiplayer online game (MMOG) market.