Package(Interface) is a collection of themes that determines the visual output and frontend functionalities of your store. An interface can be assigned on either the website-level and/or store view-level through the admin panel.
A theme is any combination of layout, template, locale and/or skin file(s) that create the visual experience of your store. Magento is built with the capacity to load multiple themes at once, therefore distinguishes themes into two large types:
Every interface comes with a theme called ‘default’ which is the main theme of an interface. When you assign an interface to your store, the application automatically looks for this theme ‘default’ and loads it to the front-end. In order to customize your store design, you can either modify this theme alone, or create a non-default theme in addition and load it alongside the default. The default theme must contain all the required layouts, templates and skins to run a store error-free and hence is the lowest theme in the theme hierarchy.
A non-default theme can contain as many or as little theme files as you see fit for your need. This type of theme is intended for use on creating temporary seasonal design changes to a store without having to create a whole new set of default theme—By creating a few images and updating some of the CSS, you can easily turn your store from a real bore to a stand-out seasonal Christmas store.
A theme consists of any or all of the following:
(located in app/design/frontend/your_interface/your_theme/layout/)
These are basic XML files that define block structure for different pages as well as control META information and page encoding.For in-depth look into layouts, read Intro to Layouts)
(located in app/design/frontend/your_interface/your_theme/template/)
These are PHTML files that contain (X)HTML markups and any necessary PHP tags to create logic for visual presentation.
(located in app/design/frontend/your_interface/your_theme/locale/)
These are simple text documents organized on a per language basis that contain translations for store copy.
(located in skin/frontend/your_interface/your_theme/)