Servo Motors are DC Motors (check out how DC motor works) with a servo mechanism to provide a precise angular motion. Pulse width modulation (PWM) technique is used to set the angle of rotation. Generally RC servo motors have a rotation limit of 900 to 1800 but servos with high rotation angles are also available.
A Servo motor with a rotation limit of 1800 is shown in the image.
A servo needs three inputs to operate - one for positive voltage, second for ground and third one for PWM.