preprocessor postprocessor

Hi,

One often hear about post-processors for CAM programs, and to this
programming dummy, it is something that ensure the NC program
generated will be understood by the Machine (is this the same as the
controller?).

Just learned that there is such a thing as preprocessor, which I
dimly understand to be something that make sure that the source code
will be understood by whatever compiler to be used to compile them
into an executable program.

Is my understanding way off the mark? Is there any thing that anyone
can add to the above that can add to our understanding of what
exactly is a post-processor? Like perhaps throwing some light on how
a pre-processor and post-processor work, and if one can dish up a
post-processor (other than for the infamous MasterCAM) even without
knowledge of the source codes?

Thanks

Peter