What's FXS & FXO? Is there any application and limitation on the interface?

FXS: 
An FXS interface provides -48 VDC power and generates ring signals, connects to standard analog telephones or PBX central office line normally.

FXO: 
An FXO interface receives -48 VDC power and receives ring signals, connects to PSTN line or PBX extension normally.

FXS interface can not be connected to another FXS interface, and must not be connected to PSTN line or PBX extension, 
Otherwise it will cause devices damaged; FXS just can be connected to digital PBX central office line.