Hy Kal,
Nice to see that you are still investigating electronics.
Here is a general diagram for a closed loop servo:
(1) In control theory there are two fundamental types of systems: open loop and closed loop.
(2) An open loop system has no feedback, so in the diagram above there is no sensor but there still may be an amplifier. Examples of open loop systems are vacuum cleaners and a spin driers.
(3) A closed loop system has a sensor which feeds information back to a servo amplifier. Examples of closed loop systems are electric drill speed controllers (speed feedback) and domestic ovens (temperature feedback).
(4) The principle of a closed servo loop is that you give it an input and the closed servo loop does its best to set the sensor to match the input.
(5) The servo amplifier compares the input with the output from the sensor and amplifies the difference to drive the motor. So yes, a servo amplifier is an amplifier which amplifies voltage or current or both depending on the motor. In fact, you can use an opamp or an audio amplifier as a servo amplifier. Note that motor here has the general sense and is a device that converts electrical energy to mechanical energy.
(6) In the above diagram, you may want to position the motor shaft at a particular angle between 0 degrees and 355 degrees. In that case you may fit a sensor to the motor shaft that has an output of 0V for 0 degrees and 5V for 355 degrees. So, for example, if you wanted the shaft to be at 205 degrees, you would command an input voltage of, 205 *5V/355.
(7) On the other hand you may want the motor to spin at a particular speed, as in electric drill controllers. You would then fit a sensor that generated a voltage output proportional to RPM.
The world is full of open loop and closed loop control systems, not just electrical, but the principle is the same. In fact, you have been a servo amplifier yourself in a closed loop control system if you drive an automobile: you control road speed with the accelerator and you monitor road speed (feedback) by looking at the speedometer.
spec