A radian is a unit of angle, equal to an angle at the center of a circle whose arc is equal in length to the radius. The radian tells you the angle measure which is also equal to the arc length. Radians are used in unit circles. You can use radians to represent the angle and arc lengths. Pi is used in the equation to find the circumference of a circle and pi is also used in radians. For example, pi radians is half of a circle and 2pi radians is a full circle. Radians can be converted into degrees and degrees can be converted to radians so the angle measure represented in degrees would equal that same angle measure if it was represented in radians. I prefer degrees because it's less complicated and they help me better understand the unit circle. Radians are more mathematically pure because they are more precise and I feel that radians are used more than degrees by most mathematicians.