Basically, responsive website Design is ‘Serving Your Website on ALL Devices‘. Responsive is an approach to web design 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.)
Google’s Matt Cutts provides a great, non-technical explanation: “Responsive design just means that the page works totally fine whether you access the website URL with a desktop browser or whether you access that website URL with mobile browser,” Cutts said. “Things will rescale, the page size will be taken into account.”
The use of mobile devices to surf the web is growing at an astronomical pace that is why there is this relatively new development to ensure optimisation for those mobile devices.
Mobile devices are often constrained by display size and require a different approach to how content is laid out on screen.