Responsive web design is a term introduced by the American developer Ethan Marcotte. It is a way of styling an HTML document that ensures the page is displayed optimally on all kinds of devices (phones, laptops, netbooks, tablets and so on).
Mainly thanks to Media Queries, a feature included in the CSS3 specification, the properties of the device on which the page is being viewed can be detected, and the page and its content adapted accordingly.
Responsive web design has three basic components: a flexible layout, flexible images and Media Queries.