Q.: Resonant Transformers

[O. Sharp]

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.-