14 November 2015
In a nutshell, responsive design describes the techniques needed to ensure that websites can be viewed properly on desktop PCs, netbooks, laptops, tablets and smartphones. Many of these devices have different screen sizes, so it's important that websites can adapt or "respond" accordingly. An easy way to think of this is to consider a shopping website which, when viewed on a PC, displays items in a three-column layout. A responsive design would typically mean that two columns are shown on tablets, while on smartphones items would be displayed in just a single column.
I've spent much of this year converting clients' websites to responsive designs. It generally takes a few days to convert an older, tabled-based design to a responsive layout; large, complex sites take quite a lot longer. Modern, DIV-based sites can generally be converted easily and quickly. A set of new images may be needed (smaller logos for use on smartphones, for example), but this is a very minor issue.
Responsive design is now an integral part of the design process: a well-coded site will not only render as expected on PCs, tablets and smartphones, but also transition smoothly between different screen sizes. For more information, Wikipedia's article about responsive design is informative, well-sourced, and offers links to other useful CSS resources.