Responsive web design (RWD) uses fluid grids and flexible images to provide optimal viewing experiences across different devices. It involves using media queries and conditional CSS to progressively enhance a website for different screen sizes rather than separate mobile sites. Key aspects of RWD include flexible grids, media queries, and flexible images and media. Popular frameworks like 960 Grid System help build flexible layouts. More websites are adopting RWD to create responsive designs that work on any device.