Since the game’s introduction in November 2004 the company has expanded to more than 1,800 employees from around 400. Almost all of the additions have been customer-service representatives to handle World of Warcraft players, helping them with both technical advice and billing concerns.

“Ultimately, what I’d like is for the user to feel like they are having a very polished entertainment experience,” said Mike Morhaime, 38, Blizzard’s president (and a gamer since he first encountered Pong in 1976). “We’d like players to associate our name with quality, so if they see a box on the shelf and it says Blizzard Entertainment, they don’t need to know anything more than that.”

The basic genre that World of Warcraft belongs to is called the massively-multiplayer online game, or M.M.O. The “massive” refers to the fact that in an M.M.O., thousands of players simultaneously occupy one vast virtual 3-D world. (In a more traditional online game like Quake or Counter-Strike, there are generally fewer than a dozen people in each arena.)

Blizzard runs hundreds of copies of the Worlds of Warcraft universe, known as servers, and there might be a few thousand players on any server at any given time. There are servers customized for six written languages: English, both simplified and traditional Chinese, Korean, German and French. Spanish is in development.

To begin, a player creates an avatar, or character, customizing its physical appearance as well as race and profession, each of which has different skills and abilities. An elf druid might specialize in healing, for example, while an orc rogue could be an expert in stealth and backstabbing. The player is then set loose in a huge colorful fantasy world with cities, plains, oceans, mountains, forests, rivers, jungles, deserts and of course dungeons.

The players can explore on their own or team up with others to conquer more imposing challenges. As a character completes quests and defeats monsters, it gains new abilities and collects more powerful magical equipment that in turn allow it to progress to the next set of challenges. Players can fight other players if they choose, but much of the focus is on teaming up with other users in guilds like the Chosen to battle automated foes.

There were massively-multiplayer games before World of Warcraft, just as there were MP3 players before Apple’s iPod. Like the iPod, World of Warcraft has essentially taken over and redefined an entire product category.