Responsive Design or Responsive Web Design (literally adaptive web design) is the ability for a website to automatically adjust to the different terminals.
Responsive Design - Why?
With the emergence of smartphones and tablets, there are a lot of available display resolutions, making it difficult to provide the best viewing conditions for a webpage. Thus, more and more sites are now available in a mobile version to facilitate reading on a mobile devices. These websites are typically accessible from a different address, such as http://m.commentcamarche.net instead http://www.commentcamarche.net !
However, smartphones and tablets have different display resolutions and it is impossible to create several version of your websites to fit each of the resolution. Moreover, the fact of viewing a page in landscape or portrait mode also changes how a website is to displayed. Finally, having different versions of a website, requires more maintenance work, involves more errors during updates and a potential loss in terms of SEO as multiple version of the site coexist.
The responsive design meets all of these problems by dynamically changing the display of the page based on the terminal resolution (using style sheets (CSS)).