This is a video showing the infinate map generation that minecraft uses, if you don't know what minecraft is go and check it out there is really too much to explain here
http://www.youtube.com/watch?v=NszGurDUlHc
(Oh before I forget, the game has changed a bit since this video was made, trees now grow much larger and can have random branch variations, some landscaping stuff has been tweaked too)
Here's an example of the stuff you can make for now to give you an idea of some depth the game has.
http://www.minecraftwiki.net/wiki/Crafting
And here's the different block types that the envoronment spawns, some however like brick, bookcases and sponges are of course not naturally spawned and at the moment cannot be made in indev or survival...yet, you can place all them however in the free classic mode.
http://www.minecraftwiki.net/wiki/File:Template1.png
The game also features mobs of peaceful creatures that spawn during the day, such as pigs (for meat) cows (For leather) and sheep (for wool) as well as agressive mobs that spawn during the night or in dark areas such as caves, mines (both natural and manmade) of which you can light up with torches to prevent them from spawning there of course. The agressive mobs feature: Zombies, skeleton archers, giant spiders and creepers (four legged creatures that act much like a proximity bomb).
There's constantly more content being added and Notch (The creator) pumps out new content every friday, in fact last night he added cows and leather armour.
What's more is it's not even finnished (You can play for free but if you purchase now you get access to the latest beta version and beta multiplayer "infdev" (The mode portrayed in the video) when it's completed (By the sounds of it within a few weeks from now)
I heartily recommend it.
