1. Virtual worlds refer to simulated 3D online environments that users can interact with and where they can socialize, play games, or engage in other activities. 2. There are closed virtual worlds like Second Life that are self-contained, and open virtual worlds on the emerging 3D internet that allow user-generated content. 3. Virtual worlds have potential applications for entertainment, social networking, and even business, but the technology is still developing and how they will ultimately be used is still uncertain.