Doctor What?

I don't understood the 'Who' in Doctor Who. It's not his name, right? He's only ever called The Doctor (and occasionally Mr. Smith). So what is it, just a simple play on words (which the show doesn't to ever tire of repeating (at least the newer ones)).

Our local library has a large selection of The Doctor and they are only slightly out of order. Which is more than I can say for Netflix.