Responsive design is an approach to building websites that automatically adapts content and layout to different screen sizes and devices. It allows a single website to be accessed from any device without needing separate mobile sites. The benefits are that it future-proofs sites, provides a consistent user experience across devices, and reduces costs by only needing to update content in one place. When deciding whether to use responsive design, it is suitable for new medium to large sites and site redesigns, but not for existing non-responsive sites or mobile-only projects.