A stepping motor is probably beyond your capabilities at this point, so I will try to explain about analog servos. There are position servos and speed servos. The position servo will rotate to a selected point and stop. The speed servo will run at a controlled speed. An example of a speed servo is a DC motor driving a DC generator. The generator output varies with speed, so if you compare the generator output voltage with a reference voltage, you can control the speed by keeping the generator voltage (speed) equal to the reference voltage. The motor speed is usually controlled by controlling the field current.
The position servo uses a potentiometer connected to a fixed voltage. The pot output varies with position and can be compared to a reference just as in the speed servo. The servo will oscillate (hunt) if the gain is too high but you can usually stabelize it by reducing the gain. There are other, more complex methods of stabelizing the servo loop.