The SpaceNavigator 3D Mouse is the smaller version of the more professional-orientated SpacePilot Pro 3D mouse.
A lot of the additional features of the professional model have been stripped away, leading to a more streamlined and cheaper 3D mouse, but one whose functionality is far more limited.
The SpaceNavigator is made up of the control nub and one solitary button. The nub can be pushed, pulled, turned and twisted to give you better control over 3D objects.
When using this mouse to manipulate 3D objects it works well, although it takes a bit of time to get completely comfortable with using it.
It is not designed to replace either the keyboard or the mouse, but to work alongside them to offer better control in 3D applications.
While the SpaceNavigator eschews the more opulent features of its more expensive sibling for a more palatable price, the lack of a lot of those extras actually ends up making it a less appealing purchase unless you're just starting out in 3D design.