Responsive design is a practice of creating web sites that automatically detect and optimize themselves to the user's device. The need for it has grown as mobile devices capable of accessing the Web have become more common.
Traditional design practice was to build websites primarily with the users of desktops and laptops in mind. This led some sites to create separate mobile versions of the sites when smartphones and tablets became popular. Responsive design attempts to implement elements useful to all types of devices that connect to the Internet and switch between them automatically in response to what the user is connecting with.