CSS (Cascading Style Sheets) is a style sheet language used for describing the presentation of a document written in HTML or XML. CSS allows developers to control the layout, colors, fonts, and overall visual appearance of web pages. By separating content (HTML) from presentation (CSS), developers can create clean, maintainable code and ensure consistent styling across multiple pages. CSS supports responsive design principles, enabling web pages to adapt to different screen sizes and devices. Advanced CSS features include animations, transitions, and grid layouts, which enhance the user experience and design flexibility.