(See the polar form in the above drawing box)
Consider a point moving on the circle. At any point of time it's position has an X component and a Y component.
The X component is A.cos(ωt) and Y component is A.sin(ωt).
A is the radius of the circle which is same as maximum value of sinusoidal waves.
From the above two lines it is deduced that a given circular motion is linked with a particular sinusoid. Alternatively we can represent a
sinusoid by a circular motion or a rotating phasor.
For a particular frequency all the rotating phasors are rotating at the same speed. Hence their relative position does not change.
In the phasor diagram a rotating phasor is represented by a complex number that takes care of both the X and Y components.