An Acronym for Foreign eXchange Office interface.
The plug on the phone that receives POTS service (ie, expects a dialtone). An FXO port uses FXS signalling.
An FXO card is an interface card that connects the fixed phone network (say Telecom or TelstraClear's network) to your computer. You can then have real calls processed with a VoIP PABX like Asterisk.