Responsive website design is an approach to designing websites that focuses on creating sites that provide an optimal browsing experience – viewing content and navigating the website with minimal panning, scrolling and resizing – across a wide range of devices (from mobile phones and tablets to desktop computers and laptops).  
The great thing about responsive design is that it can easily be added to an existing website with minimal fuss and is probably one of the most cost effective changes you could make to your website.  Having a responsive website has the prospect of boosting sales by making your site more appealing and usable for visitors using their mobile phones.