Minecraft is a sandbox-style game made by the game company Mojang. Because it is a sandbox, there is no right or wrong way to play it. The concept behind the game is that you explore and build using different kinds of blocks in a randomly created world. It has been compared to a virtual LEGO set. If you can think it, you can build it in Minecraft.
It was originally developed for the Windows PC, but has since been ported to OS X, iOS, Android, and various game consoles. Mojang developed and released a port to the Raspberry Pi as a free download.
At the command line, type
sudo apt-get update.
sudo apt-get install minecraft-pi.
From the desktop, double-click on the Minecraft Pi Edition icon, or from a desktop terminal, type
minecraft-piand press Enter.