Cool, I was asking because my current PC has all of these ports for "audio out" that are different colors...
That mess of ports are intended for a sound system that has an individual input jack for each channel or pair of channels. Such sets of outputs seem to be common on a lot of PC sound cards and motherboards, but frankly, I've never personally seen a sound system that has inputs like that--EVERY home theater system I've ever owned, looked at in the store, or helped a friend set up has optical, coax, and HDMI ports, or some combination thereof, for the surround input. Maybe cheapie computer 5.1 systems have them or something, but I've always considered outputs like that useless, and the cords would be a mess on top of it.
What kind of speaker systems work with the 5.1 surround sound? Bose?
Just check to see if the receiver you have, or are going to buy, has an optical audio in as a port.
Like acedickson says, anything that has an optical input port should work. Technically, if you want full 5.1 sound, the receiver will also need to support whatever form of sound encoding the particular DVD uses (or PCM for audio generated by something other than a DVD), but any surround receiver built in the last decade should be able to handle Dolby, DTS, and PCM audio streams, so if you're buying new it's not an issue.
And actually, if you somehow end up with a deck that only has a digital coax input, but no optical one, you can buy a little optical to coax converter box online rather cheaply. I used one to convert the other direction once, and it seemed to work fine.