The main goal of responsive mobile design is to take your application content and present it in a way that makes the most effective use of the available context, or the available screen estate. The three most essential principles of responsive mobile design are: you have the same content, with the same logical and visual hierarchy, but the representation of the content adapts to the current context.
Simple and flexible HTML, CSS, and Javascript for popular user interface components and interactions.
Producing static wireframes to design layouts for websites, web applications and user-interfaces has worked well for a long time. However, with the exponential growth of devices upon which our designs may be viewed, the use of responsive web design techniques is becoming an increasingly popular approach to handling this problem - allowing layouts to adapt to different devices, with their many varied screen sizes.
via bradfrostweb.com
Some websites now contain 'responsive images'. These scale (or crop) depending upon your screen's viewing area, so the image sizes remain appropriate whether you're looking at the website on a mobile phone, or on a huge flat screen monitor.
via designmodo.com