Sony/ Phillips Digital Interface. It's a digital audio format that transports on 75 ohm video type coaxial cable usually terminated in RCA type connectors although I have seen them on BNC connectors. There's also an optical variety of S/PDIF that's called TOSLINK.
Word Clock, well this could fill volumes but I'll try to keep it short. Word clock refers to the clock that governs sampling rate (44.1, 48, etc). Digital data streams like S/PDIF or AES/EBU also contain clock data embedded in it which digtal devices can clock to. Some devices can also accept an external word clock for sync. You'd do this for a couple of reasons, one is to syncronise many digital devices together to one master clock source. The other would be to replace this embedded clock with a more stable, low jitter clock which improves converter performance.