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). Some, 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. Communication has, until recently, been in the form of text, but now real-time voice communication using VOIP is available. For more information read the Virtual Worlds wiki.

Disney's Toontown

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Created with children as young as age 7 in mind. This brightly colored cartoon environment offers kids the chance to play games, outfit their toon avatars, own a toon house, and battle the "Evil Cog Robots" trying to take over Toontown and turn it into a bland, black and white, skyscraper-filled, fun-deprived place.

Habbo Hotel

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Habbo Hotel is a virtual world for teens, created by a company in Finland called Sulake Labs. This busy site uses a hotel metaphor as its theme complete with a "lobby" gateway and private "guest rooms."

Secondlife

external image secondlife_1.jpgSecond Life is an internet-based virtual world launched in 2003, developed by Linden Research, Inc (commonly referred to as Linden Lab), which came to international attention via mainstream news media in late 2006 and early 2007. A downloadable client program called the Second Life Viewer enables its users, called "Residents", to interact with each other through motional avatars, providing an advanced level of a social network service combined with general aspects of a metaverse. Residents can explore, meet other Residents, socialize, participate in individual and group activities, create and trade items (virtual property) and services from one another. Wiki