Responsive is a design approach that ensures websites display correctly across the plethora of devices and screen sizes that people use. Responsive design creates an optimal viewing experience on practically any device and does not require any complex server side code or alternative content.
Exactly the same content is piped to everyone, regardless of device, but the image sizes, font sizes and layout are adjusted automatically (with the option for selected areas to be hidden or shown) according to the screen size of the device being used. While there are drawbacks to responsive design, and it isn’t always the best option, it remains the most flexible, practical and scalable approach when devising web strategies for mobile & tablet.