Second Life is a three dimensional virtual world in which users control avatars that can socialize, collect items, share and learn, market products, and communicate with each other in a fantastical number of ways. Second Life is a product of Linden Lab which was founded in 1999 to be "a free online virtual world imagined and created by its Residents." There's no specific goal to playing Second Life, which allows participants to enter the virtual world with their own individualized goals and agenda. In that sense, Second Life can best be thought of as a social MMO (massively multiplayer online game).
Second Life has developed such an advanced virtual society that it has its own economic system, run on a currency called Linden Dollars. The demand for virtual services and goods within Second Life is such that people are willing to use real world money to purchase Linden Dollars.