Responsive websites automatically adapt their layout and content based on the screen size of the device being used to view the website. This allows websites to be easily viewed on all devices, from phones to desktop computers, without resizing or scrolling. The key benefits of responsive websites are that they provide an optimal experience for users on any device, require less development than separate mobile sites, improve visibility and usability on mobile, and help ensure a website remains effective as more people access the internet on mobile devices.