SPDIF is an electrical format. Sony/Phillips digital interface. It uses a phono type plug (called RCA in the US) The optical connection uses the same data format, but connects via a fibre optical connection. That is often called "lightpipe', so a search for that might help.
Both types are capable of transferring eight channels of audio simultaneously at CD quality.
Equally, both types have been somewhat superseded by USB2 and Firewire these days.
Cheers
Dave