This goes hand in hand with the growing volume of mobile/tablet users. Responsive design basically means that your page responds to different resolutions and screen sizes. My ictezy.com is an example, try re-sizing the browser window and see how the layout changes along. Hopefully you can comfortably read this post on a mobile phone, on a tablet and on a desktop monitor without manually zooming in/out.
Responsive design to a certain extent can be done by pure CSS techniques, some tips:
There are combined js/css frameworks, like: