What's difference about digital and analog signal?

For easy explanation, I use a parable example:

You got a document you need to copy to you friends. It's hand written. But at first you don't know that other class will want the document too. How will you distribute it?

Analog:
You photocopied the hand-written document and distribute it to your classmates. Then the next day other class asked your classmate to get copies of the document too, so your classmate photocopied his copy. The result is the original compared to the first and second copy are all different. The copy the other class get is blurred compared to the original.

Digital:
You typed the hand-written document and save it as .doc document. Then you emailed it to your classmates. When the other class asked you classmate for copy, he emailed the .doc document to that other class. No copy is different than the original document you typed right?

So, a headphone cable which transfers analog signal also affects the sound quality of the headphone. When it comes to analog, cable does matters. AFAIK the most expensive cable is Piccolino, 1 meter for an IEM (In ear monitoring, kind of earphone) costs about $1000, but gives the sound quality equals it's price.

Piccolino cable for JH IEM

Digital signals doesn't as matters as analog signal. Only experienced audiophiles can hear the difference between digital cables (such USB, coaxial, optical which connects transport to external DAC). Still, the most expensive digital cable doesn't beat the price of most expensive analog cable.
I recommend USB cable with ferrite cores for connecting a PC to an external DAC.