Wikipedia says:
Responsive web design is a web design approach aimed at crafting sites to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices.
Basically, responsive web design is simply this–a website design that adjusts easily to fit on desktop, tablet or smartphone browser screen.
Many smartphones render the full website on their small screen. Users have to zoom in and out to read portions of the web page. For customers it can be a frustrating experience. They have to resize the site to read different sections, and can have a hard time using small buttons that are intended for interaction with a mouse rather than for a finger on a touchscreen. Hence the development of responsive design.
A responsive website changes its appearance and layout based on the size of the screen the website is displayed on. Responsive sites can be designed to make the text on the page larger and easier to read on smaller screens. They can also be configured to make the buttons on the phone’s screen easier to press.
If you are not sure whether your website has a responsive design, try viewing it on different devices. What is your experience? Is it frustrating?