SPDIF

The standard to connect audio components ‘digital’ is based on the S/PDIF protocol.

It can be done optical (Toslink) or electrical (coax).

Toslink

You won't find many computers with Toslink.
Apple and Toshiba are exceptions to this rule.
Even a lot of Apple owners don’t know that the headphone out doubles as a Toslink out.
Likewise the microphone input accepts Toslink in.
You need a mini-toslink adapter or a mini to male Toslink cable.

If Mohammed will not come to the mountain, the mountain will go to Mohammed.
That is exactly what happens, as the audio industry most of the time offers legacy SPDIF connectivity instead of USB, especially the multi media PC's close the gap by offering SPDIF over optical or electrical.

Toslink specs of the iMac:

Jack type: 3.5 mm (1/8-inch) stereo combo
Digital audio signal-to-noise ratio (SNR): >130 dB
Digital audio total harmonic distortion + noise (THD+N): <-130 dB (0.00003%)

More about Toslink

Coax

Coax cables in general use RCA or BNC connectors. Range can be 10 – 15 meter or even more. The ones equipped with RCA connectors look exactly like a analogue RCA cable but for best results it must be a true 75 ohm cable.

A ‘digital’ cable can be used for analogue as well. More about digital cables can be found here.

Not a recommended practice but for short runs, an 'analogue' cable might do the job.

Then of course you have people who think that you can connect the digital out of a sound card with a RCA Y-splitter cable to the analogue in of your amp or active speakers.

Electronics are simple, if you make a mistake the spot is automatically marked with a smoke signal.

A quad shield RG6 (antenna cable) will do the job at a very low price. Some even run S/PDIF over 30 m RG6 with success. Something to keep in mind before you buy a Audioquest Eagle Eye with 72V DBS Digital Cable at $650,- per meter.

If your PC doesn't have SPDIF this might a matter of not having a receptacle.
Often there is SPDIF out on the motherboard.
If you don't mind a bit of DIY you can make your own SPDIF out.
http://projects.loetaffe.de/index.php?option=com_content&task=view&id=34&Itemid=12

AES/EBU

The prosumer standard is AES/EBU. It is a balanced connection using XLR connectors.

A 110 ohm cable must be used.

Length up to 300 m are possible

You can’t measure this impedance using a conventional volt-ohm meter.
Well you can but you will measure something close to zero.
This is the DC impedance

These standards refer to the characteristic impedance.
This is the AC impedance of the transmission line and is defined as:

L=inductance per unit length
C=capacitance per unit length

Wireless video transmitter

One day I stumbled upon this post: A guy connected two audio devices by sending the S/PDIF over a wireless video transmitter.