What part of motion control don't you understand? The motion part or the control part? Motion control is all about controlling motion, period.
With a PLC you can make things move by using motors or pnematic/hydraulic cilinders. To control that motion you'll need info about the motion. Servo systems give back info about the motion. Stepper motors don't need to return info, because you can determine the motion by stepping the motor through a certain number of well determined steps.
Servo systems are analog systems, stepper motors need a higher frequenty control system than PLCs can deliver, hence the need for extra, specialised modules in the PLC or even special controllers.
Regards,
