A responsive website is one that adjusts it’s size depending on the type of device you are on. This ensures an optimal user experience across all devices. It is the most cost affective option if you want your site to both look and perform well on all devices.
Mobile accounts for more than 52.2 percent of all web traffic worldwide, so it is imperative that your site caters for this majority of users. To add further importance to the need for a responsive site, it is also expected that Google will start to penalise sites that are not optimised correctly for mobile devices in the later half of this year.