Understanding motor controllers and example application
To drive a DC motor in a controllable manner you need:
1) Power stage, which is typically an H-bridge of some sort. You can build one yourself or buy an integrated solution.  
2) A controller able to drive the power stage, typically using PWM signal. It can be a programmable microcontroller or a circuit based on a popular 555 chip for example.

For a position feedback you need a position sensor. It can be: 
1) Shaft encoder 
  a) Optical 
  b) Magnetic 
  c) Potentiometer - not good for continuous rotation, only if the rotation range is limited.
  (a) and (b) can be either incremental or absolute. Absolute is good if you need to know the absolute position of the shaft without performing any homing operation. Incremental is good if only relative position is of concern. 
2) Some kind of position sensor on the end effector - could be encoders as well.
And you need something to connect the sensor to and implement the control loop. It is typically a microcontroller with dedicated program. 
And yes, you will need all of these for each motor (well, they can share the same microcontroller if it is capable enough.