Responsive web design or RWD is a web design method that automatically adapts the rendering of a web page according to the screen size or orientation of the device (tablet, desktop, netbook, smartphone, etc.). Adaptation is done by changing contents, images and text sizes and text layouts after detection of the device, screen size or, more often, browser window width.
In responsive web design images and layouts are often expressed as a percentage of the browser window or screen width.
Responsive web design guarantees a good user experience and allows costs reduction by avoiding the creation of multiple web or mobile pages as only a page with different conditional contents and layouts is created.
Responsive design is used for web contents and emails (see some particularities of responsive email design).
IAB offer an example of responsive web design:
Another example of adaption according to screen dimensions:
Image credit Miratech