WHAT ARE FXS, FXO?

FXS (Foreign Exchange Station) is the port to which you connect your telephone.

FXO (Foreign Exchange Office) is the name of the port where the cable from the switchboard can be connected.

You will mostly encounter the terms FXS and FXO in ATA (Analog Telephon Adapter) devices. ATA devices are used in VoIP applications, it is used to take the call over VoIP or over telecom at any time. In such devices there are ports where you can plug in the telephone line from the telecom, these ports are called FXO. ATA devices also have ports where you can connect your phone, these ports are called FXS.