It's a matter of representation. Do the signals represent continuous or discrete quantities? A digital signal represents a discrete quantity such as an integer

It's a matter of representation. Do the signals represent continuous or discrete quantities? A digital signal represents a discrete quantity such as an integer or symbol, or a sequence of those quantities. Digital systems possess the feature of "noise immunity," where a signal can be unambiguously interpreted due to rules that involve thresholds. For instance you can look up an oscilloscope trace of the signals on a USB or Ethernet cable, and they look horrid, but those signals can transmit information with virtually zero error. To expand a bit, since my day job involves this stuff, physical stimuli are always analog. Even the discrete energy levels of an atom make their transitions in continuous time. Yet there are good reasons to do virtually all computation in the digital domain, where "noise immunity" allows processing to occur without the introduction of additional noise, and you enjoy all of the other benefits of computer programming. These days, the job of the analog person is often to understand the physics of the quantity being measured, and the sensor, but to get a signal safely to the front end of an analog-to-digital converter. Now, the irony is that I actually spend most of my time working in the digital domain. The reason is that analysis of the digital data stream is how I know that my analog stuff is working, and how I optimize the overall system. So if you watched me work for a week, you'd notice that I actually spend a fair portion of my time coding. I just don't write software for other people to use. That's another department, and their work usually starts after mine is done.