There is a huge amount of people that play minecraft. The number of players you actually see in the game is different ass there are hundreds of thousands of servers all over the world.
If your thinking about playing find what server your friends play on and join them other wise you can find servers everywhere you look really.
Minecraft is a game in which you build and explore a world made by the server. Everything is in a block form and so you put and remove blocks from the game. While you try and survive the monsters at night.
Yep a lot of people do, Minecraft is essentially a bucket world, which is endless (you can walk in any direction and it will continue spawning the world around you) it is quite vibrant with animals dotted through-out. So it consists of the average survival in the wilderness building, creating/finding food, taming pets and mining! It is a lot of fun. You can see it here: https://minecraft.net/ and the wiki to help you with recipes: http://www.minecraftwiki.net/wiki/Minecraft_Wiki