Responsive web design (RWD) is a web development approach that creates dynamic changes to the appearance of a website, depending on the screen size and orientation of the device being used to view it. RWD is one approach to the problem of designing for the multitude of devices available to customers, ranging from tiny phones to huge desktop monitors.
RWD uses so-called breakpoints to determine how the layout of a site will appear: one design is used above a breakpoint and another design is applied below that breakpoint. The breakpoints are commonly based on the width of the browser.
The same HTML is served to all devices, using CSS (which determines the layout of webpage) to change the appearance of the page. Rather than creating a separate site and corresponding codebase for wide-screen monitors, desktops, laptops, tablets and phones of all sizes, a single codebase can support users with differently sized viewports.