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Design packages in Magento give great flexibility for designers and developers.
Let’s start with terminology. There are packages.
They contain themes. Themes contain layouts, templates, skins and translations.
Layouts are XML files that define block structure and updates for different pages and flow cases on the store.
Templates are regular PHP files that mostly contain HTML with necessary PHP tags.
Skins are CSS files and images
Translations are Zend_Translate compatible files (CSV, XML, gettext)
It is possible to use these components from different themes. Say, you have a X-mas promotion. And the only thing you need to change is skin (CSS and images).
That means you can configure a store to continue using the default layout and templates, but to use xmas theme for skin.
There’s a default theme.
If a file was requested from a theme and it does not exist there, it will be looked for in the default theme. That means that you do not have to create a copy of the whole theme, but only the files you want to change there.
Here’s some more info: Magento folder structure
More to come...