Responsive Web Design is an approach of developing a website so it adapts to the size of the screen and providing an optimal viewing experience to the reader. Responsive design takes into account user interaction across devices, acknowledging factors such as ease of reading and navigating through the website with minimal need to resize, pan or scroll. Text, images, videos and other elements will automatically re-adjust themselves according to the browser size or device, with no need to create and maintain different versions of a single website.
With Google's recent crackdown on websites that aren't mobile friendly, there's never been a better time to ensure your website is mobile-firendly from the start!