The document discusses responsive web design and mobile web development. It defines responsive web as websites that detect screen size and orientation and change layout accordingly. Key aspects of responsive design include using relative units like percentages instead of pixels, responsive images, less JavaScript, and media queries to apply different CSS stylesheets based on screen size. The document also covers dedicated mobile websites that are built specifically for mobile rather than being responsive, and initial steps for mobile web development like adding viewport meta tags and including frameworks like jQuery Mobile.