Minecraft is an independent sandbox game developed by Markus Persson. A sandbox game allows a large amount of freedom with the gameplay. You can mine minerals, craft armour, weapons and tools, construct buildings and create entire worlds in Minecraft. Each world (called a "map") is stored as a directory in the Windows program application file directory. If you've downloaded a Minecraft map from the Internet, you can play it by replacing one of your single player maps. This allows you to import the map into Minecraft and play it as if it were created by you.
Double click to open the compressed map file in your file extraction program. Extract the compressed map file contents to your hard drive, then open the directory that contains the map file folder.
Type "%appdata%.minecraft\saves" into the address bar of the Windows Explorer window.
Drag and drop the downloaded map file folder into the "Saves" folder.
Launch the "Minecraft" program. Type your username and password into the displayed text fields, then click "Login."
Click "Single Player."
Click the imported map file name to select it, then click "Play Selected World" to launch the map in Minecraft.