In my lifetime, everything's gone from being analogue to being digital. These terms are usually used when talking about electronics, but they needn't be.
I think of digital as being where a range of values correspond to another discrete value. In his book, The Digital River, Richard Dawkins says that DNA is digital. That's because even if you bend and twist the DNA around a bit, the order of the proteins stays the same, and it's the order that counts. When the DNA is copied, the copy isn't nearly the same like a photocopy, the information in the copied DNA is exactly the same, like a copied computer file.
I think that speech and writing is digital too. When I've played Chinese whispers, it's been disappointing because the phrase never changes. If one were to play Chinese pictures (I'm sure there is such a game) with art that didn't contain symbols, the change would be immediate.
This is because with speech, a large variety of sounds map to a single word. It is digital. Also with writing, a large variety of marks on paper map to a single letter. It too is digital.