Good coverage RIX!
To add to this analogy you need to think of your telephone system or home phone. The FXO stands for Foreign eXchange Office equipment meaning the connection to the telephone port or telephone company. FXS stands for Foreign eXchange Station or the actual end-user equipment ... the telephone itself. The FXO (or 'z' end) would connect to the CO Line and then transport this facility to a matching FXS device (or 'a' end) where you would connect the telephone.
An OPX (off-premise extension) is a good example of the this feature. You take the 'z' end (FXO) and connect it to your analog extension port on your PBX and then at your remote location, the 'a' end (FXS), you plug in your telephone. You now have a link connecting you up to your office at your remote location.
Several companies utilize an FXO/FXS module technology to convert POTS (plain old telephone system) connections through various networking technologies and these sites have really great FAQ sections to help you undersatnd and configure your solutions.
Cisco,
Norco Communications, and
ADC Telecommunications INC (aka Pairgain).
hopin' im helpin'
Cheers[[]]D