Postprocessor

I don't understand the concept of post processors. Anyone want
to elaborate?

That is: If I get an old Bridgeport (or any mill), throw away
all the electronics except the motors, use Geckodrive boards,
Vector CAD/CAM in a suitable box, can't I just plug in all of
this and run it?

When would I have to have a different post processor? When I use
a different board than a Geckodrive, when I use a different
motor, or what?

Please don't give an detailed explaination. I just want to know
the parameters that, when changed, require a different post
processor and why.

JerryK