Responsive design is a style of site design that causes pages to adapt to screen size and display. Responsive websites are adaptive and work similarly across mobile, tablet, and desktop devices.
A responsive site only has one URL that loads a single HTML, using CSS and media queries to format the layout around determined breakpoints. Breakpoints cause your website content organization to adjust based on screen and display width.
Breakpoints should always be based on content, never on specific devices, products, or brands. Google’s recommended approach to defining major breakpoints is to start small, designing for the smallest device first.
Responsive design is ideal for SEO, because its single URL retains its link juice across all devices. It’s also ideal for content management, since you only have to manage one website worth of content for all devices.