I started to use x-cart a little for some of my clients so everything I learn about the file structure, style design, programming etc I will continually add to this post.
Where all the design files are kept using smarty template engine header.tpl The header info of the page
Generally everything in the skin1>customer folder is to do with the front end of the store. home.tpl is the home page
Skin1>main also has a few useful files prnotice.tpl is generally where the copyright notice and footer contents goes.