Ohhh I love this question - because it just so happens I'm working on an Horary for someone right now ;-)
In Astrology, Horary is a form of prediction which can help someone answer a question that has been on their mind for some time. Horary can be incredibly insightful and very detailed on topics that we have been questioning for some time. An Horary Chart is created the moment you ask an Astrologer the question. It’s very much like a “birth” of a question. The planets align in such a way where their pattern can be analyzed and interpreted to answer that particular question with detail.
I prefer only to do Horary Readings for serious questions and the results are always unusually accurate. I always keep my Horary Charts because you can only ask a question once and if you ever need more detail on the particular question you asked you can look back on it.
Horary is just another reason why I love Astrology ;-)