Ask HN: Ways to display very specific ads?

I've been reading up on ad exchanges, DSPs and the lot the past day or two and I'm trying to get the whole picture straight in my head. What I'd like to do is display a very specific ad in real-time based on my own data analysis. For example, based on the current visitor's profile, I would want to show an ad for orange juice. How could I do that? Are there APIs for ad exchanges so that I can request a very specific category of ad?

Any light shone on this murky subject would be appreciated.