These cables transmit digital audio from one piece of equipment to another. The connectors either look like:
this (an RCA cable,
just like the analog one):
this (a
TOSLINK optical),
or
this (a mini-TOSLINK optical).
Credits
(you don't have to copy this part):
RCA
connector pic from
(http://www.soundphilosophy.ca/images/puresonicpics/ps7010A.gif)
TOSLINK
pic from
(http://content.answers.com/main/content/wp/en/thumb/d/da/300px-TOSLINK.jpg)
Mini-TOSLINK
pic from (http://www.cablesdirect.com/prodimages/FCTKMP-A_LR.jpg)