The World Coordinate System is based on the Cartesian Coordinate System. Having an understanding of this coordinate system (which many people have, despite of probably never heard of the name) is essential to understanding how the WCS (and UCS) work within AutoCAD.
By A couple of releases back, AutoCAD introduced Dynamic UCSs. Because dynamic is the opposite of static, what you get is a temporary coordinate system that changes as you move the mouse pointer over different planar faces of a 3D object. Click the Allow/Disallow Dynamic UCS button on the status bar, or press the F6 key to toggle Dynamic UCS on and off. The UCS command offers ten options to help you define a new UCS. Access the UCS command options from the Coordinates panels on either the View tab or Home tab of the Ribbon. After you select one of these UCS options from the Ribbon, follow the command prompts at the command line or the Dynamic Input prompt.