Yeah Dobro, it stands for Sony/Phillips Digital Interface. It looks just like an RCA jack. I have been told by some to only use S/PDIF cables and others have told me that 75ohm RCA cable is okay, although I have never tried it.
It transfers digital audio in stereo (I think it also transfers disc data, i.e. a CD's TOC or track numbers, track timing, etc.) with just one cable.
Lightpipe and T/DIF (TASCAM Digital Interface) look like computer cables and tranfer digital audio in multiple track formats. ADATs use lightpipe and TASCAM's DTRS (DA-88, 38) use the TDIF. They both do the same thing, but are not interchangable. They do make TDIF to Lightpipe converters (and vice versa) though.
But nobody asked about that. I am rambling for no good reason ----
Brad