Describing what
Minecraft is can be a little difficult. Even the developer doesn't seem quite sure how to describe it:
Minecraft is a game about placing blocks while running from skeletons. Or something like that..
Sadly, his description doesn't do the game justice, nor does it entice people to really find out what it's about. It's probably more accurate to say Minecraft is kind of like a first-person version of
Dwarf Fortress, except then you'd have to know what
Dwarf Fortress was for that to mean anything.
EDIT: A new introductory video has been put on Minecraft's front page. This is a lot more interesting than a video of him testing out mine carts.
You arrive in the huge world (about 8 times the surface area of the Earth, but not even close to the same depth) with nothing but your good health and will to survive. You need to gather natural resources which you can use to craft into various tools and goods, which you then use to craft more tools or for certain purposes. But be careful when night falls (or any time inside dark caverns) for that is when/where the monsters lurk! And there are more than just skeletons to run away from or fight.
This makes your first day in the game somewhat frantic, since you only have about 10 minutes of daylight to create a shelter with lights before the sun goes down and the monsters come out to destroy you. It's imperative to have lights in your shelter, since the monsters can spawn anywhere it's dark. As I said, this means outside at night and anytime inside/underground in the dark.
Here's
a YouTube playlist of X's Adventures in Minecraft which will teach you the basics on getting started in the game.
The game itself has a free Classic mode available to all, which is almost like a LEGO Land simulator. As far as I know there are no enemies and no way to die. You just place blocks to your heart's content to build interesting things.
The latest Alpha version is where the fun I described above is, but it will cost you €9.95 (about $15 USD, currently).
Both versions of the game also support multiplayer, but the Alpha version's server doesn't have all the features implemented yet and is still quite buggy. That said, this game has been wildly successful. Even though it's only in Alpha
over 300,000 people have purchased the game!
I recommend you give it a try and watch a few of the
videos in the playlist to see if you think it's worth the €10. I personally paid for it back when it didn't even have all the features of what is now called Classic mode. I was very pleasantly surprised to see it turn into such a fun game in the meantime.
http://minecraft.net/And Notch (the developer) has just announced an
upcoming Halloween Themed update with some more features and bugfixes and other improvements.
Other resources: