A theme is a collection of files that determines the visual presentation of your store. When you first install Magento, 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.
A responsive theme adjusts the page layout to fit the view port of the device. Magento’s sample “Luma” theme has a flexible, responsive layout that can be viewed from the desktop, tablet, or mobile device.