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Radians

Angles can be measured in degrees or radians. 

There are 360 degrees in a full circle.

One Radian is (180/) degrees, or about 57.296°

Consider a circle of radius r as shown on the right. If we mark off a portion of the circumference so that the selected distance around the circumference is equal to r.

This portion of the circumference subtends an angle at the centre, which has been labelled in the figure with the Greek letter theta.

Because the portion of the circumference that is enclosed by this angle is equal in length to the radius of the circle, the angle itself is called as one radian.

Here is a link to an interactive page that illustrates radians really well - it also explains why we bother having radias as well as degrees to measure angles.