Second Life is a virtual environment in which avatars -- visual representations of users or "residents" -- can interact. If you're familiar with The Sims, it's superficially similar, since it's less a game with goals than a venue for socializing. However, Second Life is radically different in one important way: The entire world -- attractions, objects, events -- is created and owned by the residents.

Haven't I heard this thing also called Linden? What is Linden?

You may have seen Second Life referred to as SL or, occasionally, Linden. The company developing and hosting SL is Linden Lab -- God, nature and government to Second Life, as its residents note (some with dismay, about which more in a minute).

Why should I care?

Because where there's ownership, there's potential for commerce. For starters.