Just to add to what P-Dawg said, two black rings on the jack is stereo, while one black ring is mono. They are normally labeled pretty clearly on the packaging or on a website whether they are stereo or mono, so you shouldn't have much trouble if you go to buy one.
Radio Shack (and others) also sell
stereo-to-mono adapters for those 1/8" (or 3.5mm) plugs. So, if you can't find a mono cable for whatever reason, you could use those adapters on a stereo cable.