People unfamiliar with Yahoo Store's proprietary programming language, RTML, often refer to it as RHTML, most likely because, well, RTML doesn't seem to mean anything, but since it has to do with web design, it may be related to HTML. RTML, however, does not come from HTML.
RTML was the template language used by the ecomerce platform from Viaweb, a company that was acquired by Yahoo in 1998. The name "RTML" (not RHTML) came from the screen name of one of the Viaweb founders, Robert T. Morris.
RTML is still the language of Yahoo Store, however, todays version has been completely rewritten from the old Viaweb version (which was completely LISP-based.)
For more tidbits on RTML and RHTML, read the Wikipedia article on RTML
Here at Y-Times, you can find books on RTML
as well as a ton of other information on Yahoo Store and custom templates.