“Responsive Web Design indicates that a web site is crafted to use W3C CSS3 media queries with fluid proportion-based grids, to adapt the layout to the viewing environment, and probably also flexible images.” [Source: Wikipedia.]
A responsive website is a website that will respond and adapt to the user’s behavior and screen size. The idea is to resize and reorder the design, adapt images, typography, columns, etc., based on screen-browser size, resolution and orientation instead of providing each device a specific website.