Simply put, it’s when a website is responsive to the device it is being viewed on. This means the layout and/or content of the web site will automatically adapt depending on the device the viewer is using.
So what’s new about that? The whole point about CSS designed websites is that they could adapt to different screen sizes but over the years designers and developers have preferred to use pixel size to define parameters.
Responsive design largely does away with pixel definitions – and has some extra wee twists in the code that can cater for a number of different screen sizes:- the widescreen desktop monitor, the smaller desktop (or laptop), the tablet, and the mobile smart phone.
Responsive sites being built on one code base, are easier to maintain and manage.
Instead of optimizing content for all of the major devices, you can have one website that automatically adjusts to screens. This is accomplished through a reaction to the width of the browser window.
In many ways it is what we should have been doing all along!