The Main World, the Nether, and the End
That's right: Minecraft has not one, not two, but three different worlds for you to explore. The Main World is the one you start out in, and where you'll be spending most of your time. As the game progresses, however, you'll find ways into the Nether, which is a dark, mysterious place full of unique resources for you to use to improve your tools, equipment, and other creations and is accessed through Nether portals. Using Nether portals also allows you to travel greater distances in the main world much faster, as it is a smaller plain.