Responsive web design is not only the latest in web design, but a trend that will last into the future. As more devices with all sorts of screen sizes and ratios are being released, it's becoming more important than ever to accomodate all these devices.
At the most basic level, responsive web design uses CSS media queries to load up different style sheets depending the width of your browser or orientation (portrait/landscape) of your device. We implemented this on Softcup.com.