Responsive web design is the future of web design. It won't be long till we see an HTML5 design from Facebook that delivers an almost identical user experience across both the desktop and multiple devices.
Responsive web design was a term coined by Ethan Marcotte on A List Apart (a long running and popular resource for web designers) in May 2010.
He coined it in response to the increasing number of devices and browsers on which to view a website, which was kickstarted with the iPhone in 2007 and has since expanded to include the likes of Android and Internet TV browsers. As Marcotte wrote, "in short, we're faced with a greater number of devices, input modes, and browsers than ever before."
Marcotte followed up with an entire book on the topic. It should also be noted that Marcotte initially advocated a specific method for responsive design, but Jeffrey Zeldman (whose company published the book) clarified in July of this year that "our understanding of "responsive design" should be broadened to cover any approach that delivers elegant visual experiences regardless of the size of the user's display and the limitations or capabilities of the device."