From Wikipedia:

World of Warcraft (commonly abbreviated as WoW) is a pay-to-play massively multiplayer online role-playing game. It is Blizzard Entertainment's fourth game set in the fantasy Warcraft Universe, first introduced by Warcraft: Orcs & Humans in 1994.[4] 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.[5] The game was released on November 23, 2004, celebrating the 10th anniversary of the Warcraft franchise.

Although its initial release was hampered by server stability and performance issues,[6] the game became a financial success[7][8] and the world's largest MMORPG in terms of monthly subscribers.[9][10][11] On March 7, 2007, Blizzard announced that the user base for World of Warcraft had reached a new milestone, with 8.5 million players worldwide.[12] There are more than 2 million players in North America, 1.5 million players in Europe, and 3.5 million players in China as of January, 2007.[13] The game has won numerous awards and recognitions, including GameSpot's Game of the Year Award for 2004.

There is also a World of Warcraft Board Game published by Fantasy Flight Games and a World of Warcraft Trading Card Game published by Upper Deck Entertainment.

The first official expansion pack of the game, The Burning Crusade, was released on January 16, 2007.