Q.: Resonant Transformers

Okay, I admit it: There are many, many things I don't know. :) And one of those things came up in a conversation today, and sheer curiosity leads me to ask: What, exactly, is a resonant transformer? The subject came up here about a month or so back - I think it was in reference to a DEC PDP-8/E power supply, but that could be a mistake - and a look through Google seemed to only bring me sites about Tesla coils, which was interesting but didn't really shed light on the subject. So I'm still curious. :) What does a resonant transformer do that a conventional one does not? Does it have usefulness outside of Tesla coils? What would it be doing in an 8/E (or whatever machine it was that was being discussed at the time)? Any enlightenment appreciated. :) -O.-