Back in the old days, which is just ten years back in the online real, people used only computers to access the Internet. In other words, the computer was the only device that had the capability to hook onto the Internet. When web designers created the layout for a website, they had to keep monitor sizes and resolutions in mind.
Even at this early stage, there were several issues that had to be tackled. Computer screens came in different sizes with various resolutions. Browsers had different capabilities and rendered HTML in their own unique ways. Designers and coders had to keep all of these in mind to create websites that looked more or less uniform on all computer screens.
And then came the mobile revolution. Consumers could now access the Internet through the Black Berry, the iPhone, an array of Android phones, tablet PCs, palm tops and even TVs. This just made the life of the web designer that much more complex.
A website now had to be optimized for a wide variety of screen which varied in size from 21 inches all the way down to tiny, 7 inch windows. Obviously, a radically new design strategy had to be adopted to stay afloat on this flood of new Internet-capable devices.
That is when the concept of "responsive web design" or RWD was born. This is an approach that aims at designing websites that look good on all devices, without having to pan, scroll or resize the page much.
The RWD concept uses proportions and percentages of units instead of absolute ones like points or pixels. This makes it easy to readjust a web page to fit well within the window of any screen, irrespective of the device being used to view the web page.
Generally, media queries are used to determine the characteristics of the viewing device and the width of the browser, and appropriate CSS style rules are used to display the web page at the right size. Relative units are used to make flexible images so that they do not exceed beyond their containing element.
A combination of client-side and server-side components can be used to shorten loading times of a web page. When browsing times are reliably lower with mobile devices, this can become an important factor in boosting speeds.
Responsive web design is more of a methodology than a specific technology. It is a set of guidelines and strategies used for web design to make pages look optimized over a wide variety of viewing devices.