If circular motion is viewed in a rectangular coordinate system (standard x, y), then even when the magnitude is constant, the direction is constantly changing. Viewed in cylindrical coordinates, the speed will now be an angular speed (radians or degrees per second), and the direction refers to the axis of rotation. Something spinning at a given frequency around a vertical axis can thus be distinguished from something spinning at the same frequency around a horizontal axis. We can also account for a change in the axis of rotation, as in the precession of a gyroscope or rotating planet.
You can find a brief explanation and a figure
here. (By the way, this is a good site for getting basic explanations of physics concepts.)