A virtual world is a computer-based simulated environment intended for its users to inhabit and interact via avatars. This habitation usually is represented in the form of two or three-dimensional graphical representations of humanoids (or other graphical or text-based avatars). Most, but not all, virtual worlds allow for multiple users. The world being computer-simulated typically appears similar to the real world, with real world rules such as gravity, topography, locomotion, real-time actions, and communication.
From : http://en.wikipedia.org/wiki/Virtual_world