A classic MOP is exactly that, a potentiometer connected to a motor so it can be adjusted remotely via simple switches for increase and decrease. They inherently provide a ramp function, adjustable with the gearing reduction on the motor head.
In more recent times, the motor and the potentiometer have been eliminated, and electronic circuits are used to provide the same function. For the generic electronic ones, you have an analog output, and several digital inputs to raise or lower the output.
For electronic circuits, the output is generally a voltage, but could also be a current signal.
The advantage of an actual motor operated pot over most electronic versions, is that it can be actually placed in a circuit to affect it's operation; ie, it isn't limited to just providing a voltage (or current) output, it can actually be used as a variable resistor.
Mechanical MOP's generally hold their last setting when power is interrupted, electronic ones may or may not.