How it's constructed mainly. In use, you can usually substitute one for the other for audio analog signals (NOT necessarily video or digital audio though), the coax may be able to send a signal farther and generally less susceptible to interference.
A SPDIF digital cable has to be a 75 ohm coax cable preferably terminated with RCA plugs that maintain the 75 ohm impedance by maintaining the coax configuration of the cable. This is similar to a good quality composite video cable. Some audio cable do satisfy these conditions but not all.