The development of technology, especially devices like Smartphones and Tablets and bigger and bigger monitors introduced a new concept in web design: Responsive Web Design.
What does responsive web design means? Let`s say that you have to build a website for a client and you want to do it based on a standard grid system, for example 960.gs. But the client also wants versions of the website for Iphone and Ipad screens too. You`re not going to build 3 different websites, instead you`re going to use a responsive grid system which adapts the website for all types of screen resolutions. This is how responsive web design works.