Responsive Web Design is a web design approach aimed at crafting sites to provide an optimal viewing experience; easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from mobile phones to desktop computer monitors). –Wikipedia
A responsive website is one that responds to the device it is being viewed on. These websites adjust automatically to fit the varying resolutions of all devices, from desktop computers to tablets and cell phones.
This is achieved by using flexible grids and layouts that use an intelligent CSS stylesheet to change the way a website looks based on the user’s preferences. This eliminates the long and expensive process of developing additional layouts of your website to accommodate each device type.