1. When you start a new game save, a world will be generated at random. It's usually very big. There will be caves, waterfalls, and even lava. Everything is is random (after you make the first world, then everything is consistent). The whole level is generated, anything afterward like forts or roads or castles you build yourself. To do this you must mine, so say you dig one dirt block. Then you get one dirt block into your inventory that you can place anywhere.

2. There is one type of creature, that if it gets close to you, it will blow up and if it's near your fort it will get destroyed or at least part of it will. The creatures do not build any forts of there own, and they are only out at night. They catch fire when the sun comes up.

3. Not sure what you mean, but mulitplayer is the same as single player for the most part. The only difference is there isn't any creatures that will attack you, and most servers give you whatever, resource wise, so you can build what you want. Nothing is built automatically, everything is done by you, or someone else on the server.

4. The game currently costs 9.99 pounds, which is around 12 dollars US while it is the Alpha stage of development. After that, the price will double.

http://www.minecraft.net/

There is FREE version called Classic if you'd like to build. Classic is different than the full Alpha game. Classic will be faded out some point.

The whole game was developed by one guy until recently. He is now in the process of hiring people.