1.5.2    Navigation Cube

The main navigation cube tracks the orientation of the actual object in the main part of the 3D view. Any operation which reorients the main 3D view will reorient the navigation cube as well. The visibility and position of the navigation cube can be changed in the Preferences dialogue, Display section and 3D View tab.

Clicking on the small cube will bring up a context menu you may use to change the type of view (Orthographic, Perspective, Isometric) and to do a "Zoom to Fit".