Minecraft is a game about placing blocks while running from skeletons. Or something like that... Currently in Alpha, but rapidly approaching Beta, Minecraft is an addictive little game where you mine out minerals, craft items from them, use those items to mine/fashion better items, and keep going until a zombie eats your brains.
You start out in the middle of a near-infinite world of blocks. Each block is about one meter (or one yard for us old fashioned types) on a side. While the depth and height of the world is limited to something like 64 blocks above and below sea level, the surface area can expand to something like 8x the surface of the Earth. In spite of this, the world generates very quickly. Since your top speed is walking, the game has plenty of time to generate new chunks of land on the fly as they come into view. You can walk until you get bored. Or until a creeper creeps up on you and explodes.
terrain that Minecraft generates is quite interesting, even before you start building on it. I've been known to create and destroy a few dozen worlds in a sitting just to see what it will do. While there are always mountains and oceans, there are sometimes rolling hills, sometimes sandy beaches, and sometimes deep ravines. Once in a while, you get floating islands, lakes, waterfalls, and lava. That's just on the surface, mind you. Underground, you can expect cave systems, mineral deposits, magma, and the occasional dungeon. What's a dungeon? I'm not sure. I haven't found one yet.
If all you did was explore and build in this world, it would be interesting. This game also features monsters, which adds to the fun. So far, I've seen zombies, skeletons, spiders, skeletons riding spiders, and creepers. What's a creeper? It's a giant green exploding penis monster. No, I'm not kidding.
Of course, not everything is out to kill you. There are also chickens, cows, and pigs. These can be killed to provide feathers, leather, and ham, but you'd never want to, because these animals are just so cute. And you just know that they'd never do anything to hurt you. Get away, pig. I'm trying to type here. No, I won't pet you. Get away. Hey! Quit pushing! I'm standing right next to a cliff! Damn you, bacoooooooooon!
You start off the game with nothing in the way of tools or supplies, which is okay because you have the raw strength needed to tear trees apart with your bare hands. With this wood, you can build a workbench or crafting table. From here, your first primitive wood tools will emerge. A wooden pick won't last long, but it should let you mine out some coal and chunks of raw stone. A bit of coal on the end of a stick makes a passable torch. You can use the stone to make a stone pick and other stone tools. These will do nicely until your find some metal deposits. Metal tools last longer and make your work go faster, but you can't always count on there being plenty of metal around. Metal also makes the best weapons and armor.
When you die (and you will die) you don't really die. You just go back to the very first place where you first entered that world. Unfortunately, you do so without all your equipment and supplies. Anything you were carrying on you at the time of your death is now scattered around your place of death. The first impulse is to run right back to that place, but whatever killed you is likely to still be there. You might need to stock up on supplies first.
This adds a bit of strategy to the game. You can only carry so much with you at a given time. You don't want to carry everything you own with you, as you don't want to loose it all down some deep hole. On the other hand, if you have to go back to your base every few minutes to get more supplies, you'll never really explore the world in which you now live.
Of course, with a name like Minecraft, naturally you're expected to do a lot of mining. Some players like to dig a mineshaft near or even inside their bases. From there, they dig down deep into the bowels of the Earth, sending shafts in scientifically-determined patterns, searching for every last mineral deposit. If they don't see a lot of the world, well, that's the price you pay for being a good miner. Other players like to explore the countryside looking for natural caves and caverns. These players spelunk through the caves looking for minerals. If they miss a few things or get eaten by spiders, well, that's the price you pay for being an adventurer.
Me, I get bored too quickly to be a professional miner, but I'm way too chicken to go exploring. I tend to just mess around until I get killed or something goes horribly wrong, then I generate a new world and start over.
The crazy thing is that I never intended to play this game. The game was free for a weekend a while ago, so I tried it. It was okay, but I couldn't see actually paying money for it. I figured I'd play until the free ran out, then forget about it. Then I decided to watch a few Minecraft videos. The next thing I knew, I was shelling out $15. (Actually, the price is 20 Euros, but it's half off right now, which in American dollars is about $15.)
So, unless you have 20 Euros and a lot of free time to kill, I suggest you most definitely do not run over to the official Minecraft site and buy the game. Do not under any circumstances read through the Minepedia. Do not watch the entire series of X's Adventures in Minecraft. Do not browse through the Official Picture Thread. And, whatever you do, do not check back here for updates.
I'll be too busy pushing pigs off cliffs.