A servo is defined as an automatic device that uses an error correction program to correct its motion . The term servo can be applied to systems other than servo motors; systems that use a feedback mechanism (such as an encoder or other feedback device) to control the parameters of motion. Typically, when the term servo is used, it applies to "servo motors", but it is also used as a general control term, which implies the use of a feedback loop to position the item.
A servo mechanism may or may not use a servomotor. For example, a home furnace is a servo controlled by a thermostat. Once the set temperature is reached, a feedback signal instructs it to turn off; making it a "servo" in nature. The term "servo" describes more functions or tasks than a specific product line. For the purposes of this guide, we'll be talking specifically about servo motors for motion control applications.
Servo motors can be DC, AC, or brushless DC motors combined with position sensors; in most cases, digital encoders. Servo motors are typically chosen when the application requires a high degree of confidence that the servo motor and drive system will closely track what is being requested. Servo motor systems typically cost more than stepper motor systems due to the feedback sensors and processing electronics of servo motors.