Let us have a look at a point $P_{\alpha}$ on the unit circle obtained after rotating the point $A=(1,0)$ about the origin over a certain angle of rotation $\alpha$. In case of a positive angle of rotation $\alpha$ we rotate anticlockwise; a negative angle of rotation $\alpha$ means by convention a rotation in the clockwise direction. A full anticlockwise turn measures $360^\circ$. The below figure shows some positive and negative angles of rotation.

A rotation angle is usually specified in degrees. But with the unit circle, we can also introduce the angular measure radian, abbreviated as the unit $\mathrm{rad}$. For points $A$ and $P_\alpha$ on the unit circle, the central angle is $AOP_{\alpha}$ in radians equal to the arc length of the corresponding circular arc $AP_{\alpha}$. Angle $\alpha$ in degrees and arc length are in essence interchangeable positive quantities.

The entire unit circle has circular arc length $2\pi$ and a full anticlockwise turn measures $360^\circ$. It follows directly that $2\pi\;\mathrm{rad}=360^{\circ}$, so $\pi\;\mathrm{rad}=180^{\circ}$. To convert degrees to radians and vice versa, you can use a ratio table.

$$\begin{array}{r|c|c|c|c|c} \mathrm{degrees} & 360^{\circ} & 180^{\circ} & 90^{\circ} & \frac{180^{\circ}}{\pi} & 1^{\circ}\\ \hline \mathrm{radians} & 2\pi & \pi & \tfrac{1}{2}\!\pi & 1 & \frac{\pi}{180} \end{array}$$ In the figure below you see the unit circle with indications for both scales.

For rotations, the angle of rotation can also be greater than $360^{\circ}$ ( $2\pi$ radians). For the final position of rotated points it does not matter whether you are add or subtract integer multiples of $360^{\circ}$ (or $2\pi$ radians).