What is "Responsive Design?" Responsive design means that the same HTML (page) is delivered for the same website address (URL) regardless of the device display; desktop, tablet or mobile. The difference is that the display will be rendered differently based on the end user's display size. Responsive design enables the website being viewed to fit the size of the display.
Why is this important? There are a number of different reasons, but probably the most important is that "Starting April 21, 2015, your site's mobile-friendliness will be considered as a Google Search ranking signal (Google)." In essence, non-responsive websites will be demoted in search results.
Our current focus is to provide websites that are designed to be responsive. The pages you are viewing now, as an example, reflect "responsive design" technology in action.
To illustrate the point of "responsive design", grab the outside of the desktop browser window with your mouse and change the size of the display. These pages will automatically "respond" and adjust to fit the size of the browser window.
Next, try viewing these pages with any mobile device. The size of the pages also respond to the size of the mobile display.