There are 3 ways to measure angles, the most common way up until now has been to use degrees. As we all know there are 360O degrees in a circle. however this number was picked arbitrarily long ago and has no real value.
Note: This is why the gradian was invente (to decimalise angles), however you do not need to know anything about gradians.
In core 2 you are introduced to radians, they will be used on and off for the remainder of the A-level maths course. There are 2π radians in a circle. Although this might seem another arbitary decision it is not. It is the angle at which the radius is equal to arc length. This means that Radians are actually a number.