Motors come in many different varieties for different applications. The term .servo motor. Doesn't.t really apply to the motor itself, but rather the way in which the motor is used and controlled. In a position servo motor application (henceforth just .servo motor.), the idea is to hold the target load (generally attached to the motor shaft through a series of gears for speed and torque adjustment) in a given position.
To accomplish a servo motor function, positioning information must be obtained from the output of the motor to provide feedback for the control system. This can be in the form of a potentiometer attached somewhere in the gear train, a hall effect sensor monitoring passing teeth on a metal gear, an encoder (optical or magnetic) mounted directly to the motor, or any other such sensor which can provide position feedback of the motor shaft or connected portion of the gear train.

One of the most common uses is in automotive applications. You will find that system on the throttle body of almost all modern cars. The sensor is usually attatched to the shaft and the motor handles the "butterfly" inside the throat. Precision is the key to most servo motors. You can also have worm drive action to pull or tug on diaphrams.