Your Store’s Theme
A theme is a collection of files that determines the visual presentation of your store. You might think of a theme as the “window dressing” of your store. When you first install Magento Open Source, the design elements of the store are based on the “Default” Theme. In addition to the initial default theme that comes with your Magento installation, there is a wide variety of themes that are available “off the shelf” on Magento Connect.
To learn advanced techniques such as how to use themes, skins, layouts, and blocks, see the Magento Design Guide.