For my reference:
FXO: Foreign eXchange Office – This is the plug on your phone. It receives Dial Tone, Current and Ring Voltage. If you have a card that does this, you can plug it into your wall jack and receive calls.
FXS: Foreign eXchange Subscriber – Service from the phone company, provides Dial Tone, Current and Voltage. If you have a card that does this, you can plug your normal phone into it.
In terms of VoIP (asterisk in my case), if you want to receive POTS calls into your system you need an FXO. If you want to plug in a normal phone and make VoIP calls you need an FXS.
Both terms from the POTS (Plain Old Telephone Service) world.