It is my understanding that digital cable is able to transmit the HD signal. While the older cable method cannot. Not all channels offer HD yet, nor are they required to until 2008-09, I think.

HD is a way for big business to contol the signal and prevent recording/copying, or making recording more difficult. I know that's not a good explanation. I don't always listen closely. This is my opinion and I am not an expert. But I do consider myself a professional tv viewer.