CSS (Cascading Style Sheets) is a language used to style and format the layout of web pages. It works alongside HTML to control the appearance of elements, such as colors, fonts, spacing, and positioning. CSS enhances user experience by enabling responsive designs that adapt to various devices and screen sizes. There are three types of CSS: inline, internal, and external, with external CSS being the most efficient for managing styles across multiple pages. It uses a rule-based syntax, where selectors target HTML elements and properties define their styles. Advanced features like Flexbox and Grid help create complex layouts easily. CSS supports animations and transitions, making websites interactive and visually engaging. It separates content from design, ensuring cleaner, reusable, and maintainable code. By caching external stylesheets, CSS improves website performance. Overall, it is essential for creating modern, professional, and accessible web designs.