a question about this motor and servos ?

hello guys,
i have this dc motor system from an RC car, so what i want to do if there is anyway of using this system as a servo which mean controlling it by a potentiometer for a specific position like the servo does ?

here are the pictures

Well you'd need a potentiometer, a motor driver, and then a PID loop to do positional
feedback. And some way to set commanded position. All doable with an Arduino
and motor shield!

MarkT:
Well you'd need a potentiometer, a motor driver, and then a PID loop to do positional
feedback. And some way to set commanded position. All doable with an Arduino
and motor shield!

ok i have a potentiometer of course and L293D chip for motor drive, but what is exactly a PID loop ? and what do you mean by some way to set commanded position ?