BTS is a template system for osCommerce, and osCMax ans CRE Loaded also use BTS as template system.

Tempalte Files
main_page.tpl.php - This is the bread and butter of your template. It sets up how your template will work and be displayed.
stylesheet.css
content/ - This is where the pages go for all templates. All templates pull there pages from here.
boxes/ - All of your side boxes will go here
images/