The answer's appreciated, but I'm afraid I didn't ask a very good question. What I was really getting at was, from the perspective of an EE, what would make an expert call one type of circuitry digital and the other analog?

Clearly the output signal has to be 0 or 1, which must mean the signal is boosted or reduced to an appropriate level (assuming we not trying to encode multiple values onto a signal; I'm just talking binary). So I guess the signal has to be captured (latched?) then modified. I don't think a clock is absolutely necessary as there have been asynchronous digital CPUs, but the presence of a clock clock is very typical as well.