According to Wikipedia, responsive design is a web design approach aimed at crafting sites to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from desktop computer monitors to mobile phones). Responsive design is becoming very popular since it doesn't require the designer to generate multiple interfaces to a site for mobile, tablet and desktop. Responsive design utilizes the latest HTML and CSS methodologies to automatically adjust the design to the viewport.
It’s all about keeping the features visitors want, and none of the things they don’t. There’s no room for clunky graphics, confusing navigation, or pages locked into desktop-only resolutions. Mobile users want their Internet lean, clean, and sized for their screen.
Responsive design requires an experienced web designer with knowledge of the technologies and pre-planning for optimizing the viewing and navigation experience of the site. It's also appreciated by search engines and is starting to put quite a bit of distance between old-school site design and the latest and greatest designs. WhoIsHostingThis has put together this informative infographic on responsive design and the checklist for a successful implementation.