Second Life is a virtual world. Linden Lab creates the software and manages the servers that host this world. Users have crafted all of the objects within Second Life (e.g. clothes, houses, cars) to be used by their 2 million avatars.
To learn more, you can watch part one and part two of the talk I gave about science education using Second Life at the American Association for the Advancement of Science. For details about what we are doing at Elon, you can read Second LIfe’s Role in a Curriculum, an article in Campus Technology.