The Viewport
The Viewport is where you see the real-time updates of the changes you make to your terrain. When you move the camera, link a node, or change a node parameter, the results are seen immediately in the Viewport.
See Working in the Viewport for details.
The World browser
This window displays a Google Map view and allows you to select any region on Earth, and to download its relief.
See Working in the World browser and Using MapBox for details.