CSS (Cascading Style Sheets) is a styling language used to control the visual appearance of web pages. It allows for the separation of presentation from content by defining style rules for HTML elements, such as colors, fonts, margins, spacing, and layouts. With CSS, developers can create attractive and consistent user interfaces while ensuring that pages are responsive and adapt to different screens and devices. By using concepts like selectors, pseudo-classes, and grids, CSS offers immense flexibility to customize the design of websites, thereby enhancing the user experience.