Responsive design is a fairly new approach to design and front-end development which allows a page to be rendered using different css rules depending on the viewport of the visitor’s browser. While it doesn’t work well for all cases, it can be a great approach to creating a site that’s pleasant on all devices while serving from a singular source, allowing development to abandon the practice of user-agent checks via an htaccess file that redirect to an independent mobile site.
For an example of responsive web design, resize your browser window with this site loaded (change the window width drastically, assuming you’re on a device which allows resizing) and pay attention to the changes in page layout.
Other examples of responsive design:
Including response.js will allow responsive designs to work in IE6-8.