Since I always do what xkcd tells me to. I immediatly tested the old version of Minecraft, which is free, and then bought the beta. Yes, it's still in beta and yet they have over two million paying users.
The home page describes it as:
"Minecraft is a game about placing blocks to build anything you can imagine. At night monsters come out, make sure to build a shelter before that happens."Which describes the pretty well. Depending on your imagination and persistence you can build anything, it's like LEGO that way. You can either go the drug-induced but orderly xkcd-way, or you can go all out crazy and build a scale model of the USS Enterprise (that's United federation of planets Star Ship Enterprise in case you thought anything else). The other way to go in the SF - Fantasy spectrum would be a recreation of Tolkien's Middle Earth, but no one could be that crazy, or could they?
The alternative is to play the survival mode, where monsters come at night. It starts all peaceful with sheep and cows milling around in a serene, sun-lit landscape. Then the sun goes down leaving the world pitch dark, and then the monsters come. First you hear the clacking of bones, or the groaning of zombies, and then you die. If you know this you build a house and light it with torches before night comes, then you are safe.
By the way, parents of small children don't always appreciate your explaining this aspect of the game to their children. Apparently it makes them afraid of the dark, or makes them excitedly tell their younger siblings that the monsters come at night, which amounts to the same thing.
So, that's about it.
Man modestly mines minerals.
Moonlight marks midnight.
Man-eating monster mangles.
Mutinously mutes machine.
Mired mind must muster motion.
Meets miserable methods-mountain.
Meekly manages more Minecraft