RTML is defined and described on Wikipedia. I am not sure who posted the entry but it is on the whole  accurate and informative. I would disagree with the notion that RTML is being phased out. It is true that Yahoo Store is now part of Yahoo Merchant Solutions, which adds the ability to build an e-commerce store in the web hosting account rather than the store editor. However, many new merchants continue to use the store editor and hence the RTML template system. In recent months we have seen the introduction of new RTML operators, which tells me that RTML is anything but finished.

I recommend also Paul Graham’s background essay on RTML and Viaweb.