Actually this is not really game. It is a virtual world, where users can create content.
You can find games within the world, but they are somewhat limited, as the simulator for a given area can't do unlimited things.
If you buy a subscription, You're allowed to buy land. An area of 512mē is free. For anything larger You have to extra on a monthly base. You can even "buy" Your own simulator, that's a junk of 256m x 256m.
At first You may think that 512mē is a lot, but You'll soon want to have a larger area. Why? Because You can only have a limited number of "prims" per area (
Wikipedia). A prim is an object that can be created in that world, combined with scripts and events.
Actually a good start is the
Wikipedia entry.