CSS (Cascading Style Sheets) is a language used to style and format the layout of web pages. It controls how HTML elements are displayed, including their size, color, spacing, and positioning. CSS enables developers to make web pages visually appealing by applying various styling rules to HTML elements. With CSS, developers can create responsive designs, ensuring websites look great on different devices and screen sizes.

CSS allows for separation of content (HTML) from presentation (style), which promotes cleaner, more maintainable code. It offers powerful features such as selectors, media queries, and CSS Grid, enabling developers to create flexible layouts and dynamic effects. By mastering CSS, web developers can create visually stunning websites with a great user experience.