Responsive web design (RWD) is an approach to web design aimed at crafting
sites to provide an optimal viewing and interaction experience—easy reading and
navigation with a minimum of resizing, panning, and scrolling—across a wide
range of devices (from desktop computer monitors to mobile phones).
Responsive web design is becoming more important as the amount of mobile traffic now accounts for more than half of total internet traffic. This trend is so prevalent that Google has begun to boost the ratings of sites that are mobile friendly if the search was made from a mobile device. This has the net effect of penalizing sites that are not mobile friendly.