Circular motion as the name suggest is the motion of an object in a circular path.Examples of circular motion is the motion of a car around a roundabout, a satellite around the earth or a stone tied to a string being whirled by the hand.
Centre of rotation
In fig 1 below you can see a circular path and a person is moving in that circular path.
The person would move in such a way that at all time it would remain at the same distance from a point that we call the centre of rotation. It is usually indicated by the symbol O in a diagram as you can see in fig 1.
Radius of circular path
As you can in Fig 2 I have draw the person at a point A and two other points B and C that the person would be during its motion later on. As is shown in the diagram the person would be at the same distance from the centre of rotation O. This distance is known as the radius of the circular path and is denoted by the letter r.
In linear motion the movement is describe in term of the displacement of the object. In circular motion the movement of the object is describe in term of angular displacement. The angular displacement is simply the angle covered by the object during the circular motion.
In the first diagram of fig 3, you can see a person moves from point A to point B. In so doing it covers and angle θ of 120o or 0.66 Pi rad. Hence the person would have an angular displacement of 120o or 0.66 Pi rad. In the second motion the person starts from point A and moves along the circular path and finally back to A. In so doing the person covers an angle of 120o or 2pi rad. Thus the angular displacement would be 120o or 2pi rad