Digital cable v. standard interconnects

This may sound somewhat naive, but what is the difference between a "digital coax cable" and standard RCA-type interconnects? Is it truly necessary to connect a digital source with a digital cable, or will any decent IC work as well?
The digital is a 75ohm cable, whereas the RCA is something like 53 or 43ohms (I don't rememebr). Yes it matters to use the right one.
Truly necessary, no. Recommended, YES.!
Ezmeralda is correct but all cable impedances are "nominal". In other words, they can vary with frequency.

You can actually use any interconnect as a "digital" cable. Since "digital" is basically an RF based signal, i would recommend something that is shielded. A simple twisted pair WILL work but it is also leaking ( more like "spraying" ) signal around the room. You will notice a difference in sound between cables when doing this if your system and hearing are up to the task. Sean
Van den Hul carbon cables are said to be usable as digital cables in their literature. I use the Second (shielded version) as interconnects. I tried one as a digital cable but much preferred my Cardas Lightning digital cable.
Since going to a separate tranpsort and DAC, I've used Kimber Illuminations D60, MapleShade Golden Double Helix and Cardas Lightning L15, thinking that a 75 ohm cable was critical. A week ago I received a fully cryo'd Audition Package of Virtual Dynamics cables and cords. The first thing I noticed was that the digital cable was not differentiated from the RCA interconnects. An A'goN member who is familiar with the VIrtual Dynamics line told me that the two are interchangeable. Well, after listening to my system through an all Virtual Dynamics setup for over a week, I am just amazed at the improvement. The sound is better in all respects. Once the Auditions are fully burned I will try reinserting some of my other digital, speaker and interconnects back into the mix just to see how they fair, but for now I am far happier than I ever expected to be.
With my $3k system (all used .. about a $6k system new) I could not hear any difference between a Canare digital cable and a good quality home brew RCA interconnect. I even could not hear a difference when I swapped in one of the interconnects which came with my marantz CD player.

so I happen to think that in an inexpensive system, like mine, that digital cables don't matter. Save your money for the analog interconnects ... that's where the difference is.
I'm no techie, but a true digital cable is supposed to have a uniform impedance of 75ohms from tip to tip. Few, if any cables actually achieve this spec, which probably accounts for the wide variation in sound and opinions as to whether this matters or not. Some analog interconnects, when used as a digital cable, may brighten or darken the sound and can negatively affect the soundstage. If you want to play it safe and minimize your investment, you can get a half way decent canare digital cable for around $20 on Ebay.
No digital cable with RCA termination will exhibit true 75 ohm (nominal) impedance. RCA connectors were designed for audio rather than radio frequency (RF) use. They do not exhibit a constant 75 ohm impedance at the RF frequencies used by the digital signal.

Ideally the digital cable, processor, and transport would use BNC connectors. Some manufacturers (i.e. Wadia) supply their gear with this connector in place of the RCA.