Responsive web design (RWD) creates a system for a single site to react to the size of a user’s device—with one URL and one content source. A responsive website has a fluid and flexible layout which adjusts according to screen size.
It offers an optimized browsing experience.
Basically, your website will look great and work well on a desktop (or laptop), a tablet, and a mobile phone’s browser.
Responsive web design offers us a way forward, finally allowing us to design for the ebb and flow of things.