Mobile devices have quite different hardware characteristics compared with desktop or laptop computers. Their screens are usually smaller, obviously, but they also usually automatically switch the screen orientation between portrait and landscape mode as the user rotates the device. They usually have touch screens for user input. APIs like geolocation or orientation are either not supported on desktops or are much less useful, and these APIs give mobile users new ways to interact with your site.
Responsive Web Design is a term for a set of techniques that enables your web site to adapt its layout as its viewing environment most obviously, the size and orientation of the screen changes. It includes techniques such as: 1. Fluid CSS layouts, to make the page adapt smoothly as the browser window size changes 2. The use of media queries to conditionally include CSS rules appropriate for the device screen width and height