Second Life is the leading 3D virtual world. It's a space where you can be whoever you'd like, build and sell whatever you can imagine, and have fun with others from all over the globe while you explore unique virtual environments, listen to live music performances, play games, shop in the world's largest user-generated virtual goods economy, and much more.
Second Life is in some cases also a game, but further more its a gigantic platform for bringing our imaginations to life :)