User Interface package

Provide skins and layouts to the system. It does not need to be instantiated in order to be used. If you attempt to uninstall an UI package that is used by a master page, the uninstall job will fail with an error. The package includes CSS, layout XML, images, javascript, favicon and other files needed for the proper organization and presentation of the user interface on master page basis.

You can read more about the user interface package topic on the "UI Reference" section:

1. Creating UI package from blank
2. Customizing existing