Prosopagnosia is an inability (usually partial) to recognise faces. Please don't say "oh yes, me too, I can never remember names either". That's not what it is. The thing I'm bad it is the earlier stage, telling whether a face belongs to someone I know or not: I (often) don't have the (reasonably) reliable sense of familiarity which is what sets most people trying to remember a name. Once I realise that I'm supposed to know this person, I'm not exceptionally bad at working out/remembering who they are. So if I see you in a context where I expect to see you, and not too many other people, you're unlikely to notice that anything's amiss. If I pass you in the street, you may think that I'm "miles away" as I walk past showing no sign of recognition. (And indeed I may be, but that may not be the explanation! Even if I'm trying hard to work out whether I know you or not, I probably can't tell.)
It isn't that I don't bother to learn to recognise people. I try hard, often. I've tried all kinds of tricks, including taking photographs and studying them daily.
It isn't, usually, that I'm daydreaming instead of looking at people I might recognise, though this is often a socially acceptable "explanation".
Although prosopagnosia is called "face blindness", it of course isn't that I don't see faces; I don't think I'm any worse than average at interpreting facial expressions, either.
I'm at least as good as average at recognising people by their voices, their walks or the backs of their heads, I think. I'll often walk away from someone and look at them from a distance if I think I might be supposed to recognise them and their face is confusing me.
There are some people I always recognise, and I do get better at recognising a given individual over time, I think, just very slowly.
This page about recognising stones is the best explanation I've seen of what it feels like.
PS Of course this isn't a new idea... here are a few links: