From CNC.xyz Wiki
A Servo Motor is a type of motor that operates in a feedback loop by using an encoder to send signals back to a computer to report its position. Servo motors are most commonly found on high end CNC machines because they can operate at higher speeds and are commonly more powerful at a lower price compared to stepper motors. While DC servo drives can be found relatively inexpensively, there is considerable knowledge required in order to get them working properly for a CNC Machine.
See also Stepper Motor.