One of the cornerstones of responsive design is the media query. The @media directive provides a mechanism for serving different styles to different devices with different characteristics. Sass both supports and extends the @media rule.
One of the more important questions you’ll have to answer in a responsive design is where to set breakpoints. Originally designers chose the dimensions of popular devices as a good place to add them. If the iPad is 768px × 1024px then 768px and 1024px might make for good breakpoints or so the thought went.
Flexible layouts work well to a point, but there comes a time when we need a more drastic change than simply allowing design elements to expand and contract. There comes a point where we need the ability to make larger changes in how elements flow within a design. Enter media queries. Now with official W3C goodness.