Responsive design allows a single website to be accessed from any device by dynamically changing the layout depending on screen size and device capabilities. It uses fluid grids, flexible images and CSS media queries so that as the screen size changes the content can reconfigure to provide an optimal viewing experience. Key elements are the fluid grid that adjusts content as the page size changes, flexible images that scale appropriately, and media queries that apply different stylesheets based on screen properties. This allows a responsive site to seamlessly transition between desktop and mobile formats.