From Linden Lab:
Second Life is a virtual world - a 3D online persistent space totally created and evolved by its users. Within this vast and rapidly expanding place, you can do, create or become just about anything you can imagine. Built-in content creation tools let you make almost anything you can imagine, in real time and in collaboration with others. An incredibly detailed digital body ('Avatar') allows a rich and customizable identity. A powerful physics simulation running on a backbone of hundreds of connected computers and growing with the population allows you to be immersed in a visceral, interactive world that as of November 2005 holds more than 80,000 residents. The ability to design and resell 3D content, combined with the ability to own and develop land and a microcurrency, which can be exchanged to real money means that you can build a real business entirely within Second Life.