A radian is the angle at the center of a circle that cuts off an arc exactly as long as the radius. One full turn is 2π radians, so 180° = π radians. To convert degrees to radians, multiply by π/180°. To convert radians to degrees, multiply by 180°/π.
Coterminal angles are angles in standard position that share the same terminal side. To find an angle coterminal with θ, add or subtract 360°, one full turn. Every angle has infinitely many coterminal angles, given by θ + 360°n, where n is any integer.
An angle in standard position has its vertex at the origin and its initial side on the positive x-axis. The terminal side is where the rotation stops. Turning counterclockwise makes a positive angle, and turning clockwise makes a negative angle. The quadrant where the terminal side lands helps describe the angle.