This document discusses responsive web design. It begins by defining responsive design as using HTML and CSS to automatically resize and reformat web pages to look good on different devices. It then explains why responsive design is necessary today due to the rise of mobile usage. The document outlines how to check if a site is responsive, the benefits of responsive design like improved user experience, and the major components used to build responsive sites like meta tags, CSS media queries, grid systems, and frameworks. It concludes that responsive design improves the user experience by avoiding unnecessary resizing or scrolling across devices.