CNC - The Basics

I'm new to the idea of converting a machine to CNC. Is there a web
site which explains the basic components (and different
combinations) of a CNC system.

I know the front end consists of a computer program that generates
G-Code (possibly interpreted from a CAD program) and outputs the
individual motion controls through the printer port. On the back end,
stepper motors or servos drive the various axis of the machine which
will perform the desired work.

My question(s) is, what is between the front and back ends, other than
a driver for each motor/servo. Where do the relays and power supply
fall into this process? Is there any other required components other
than cables? Is there a "black box" that covers everything between
the computer and the motors/servos (i.e. Does the PMDX Motion
Management System require anything else)? How much difference is
there between a system which uses servo motors or steppers?

Thanks,
Rick