A question often asked. Well, the short answer according to Wikipedia:

The Metaverse, a phrase coined by Neal Stephenson‘s science fiction novel Snow Crash (1992) constitutes Stephenson’s vision of how a virtual reality-based Internet might evolve in the near future. In Snow Crash, the Metaverse can be accessed through public-access terminals in reality. However, using them carries a social stigma among Metaverse denizens, in part because of the low visual quality of the rendered avatar; the Metaverse representations of a user in virtual-reality. Entering the Metaverse through a private terminal, making it possible to customize your avatar, is possible from almost any location using portable devices. In the Metaverse, status is a function of two things: access to restricted environments (such as the Black Sun, an exclusive Metaverse club) and technical acumen (often demonstrated by the sophistication of one’s avatar). The Metaverse is frequented mainly by the upper and middle classes.

The term itself is now becoming a focal point for all efforts to build virtual worlds; with real economy. In many ways, each of the worlds out there (second life, There, IMVU, and others) have a different degree of Metaversness. The cross between a game (like World of Warcraft or Guild wars) and a Metaverse probably has to do with real money (like Second Life or Entropia) and ability to create. But that will be the topic of another post.