MAXNC has some info on there site. They offer th only closed loop stepper system I'm aware of.
Basically, with an open loop stepper system (which most everyone uses), if or when you lose steps, you have no way of knowing it. The machine keeps on cutting away, whether it is in the right position or not.
With a closed loop system, the steppers have encoders, which make sure the stepper is in the right position at all times. If you miss a step (or steps), the software driving the machine will know from the encoder that those steps were not made, and will give the motors more steps to get back into position.