Second Life is a video game that exists in real life, and is an online virtual world developed by Linden Labs and initially released July 23, 2003. As of 2014 it has about one million regular users as per the word of Linden Labs. Though it has many elements in common with Massively Multiplayer Role Playing Games LL has asserted that Second Life is not a video game.
The game is free to play either through LL's own client (Or "viewer" as its known), or through third party viewers such as Singularity. Players, or Residents as they're called, create avatars and explore the various locations within Second Life, making friends, attending clubs, creating objects, and doing whatever else they can think of. Objects, animations, scripts, and a myriad of other content, including even virtual land, can be bought using Lindens (L$), the game's currency. Lindens can be earned from playing chance games, holding various jobs provided by other residents - usually something involving virtual clubs such as dancing, hosting, DJ'ing, and even being an escort - or by purchasing Lindens using real life money.
Second Life tends to attract the attention of furries due to the ability to purchase objects, skins, clothing, and other features that can effectively transform their human avatars into animal avatars.
Second Life is parodied in the Spectral Shadows universe through the game Another Life that is featured in Serial 11. The idea of Furries using and creating Furry avatars is also parodied, except its cartoon animals making Human avatars and having a "Pure Human Fandom" (itself being a parody of the Furry Fandom).