CSS (Cascading Style Sheets) is a stylesheet language used to define the presentation and layout of a web page written in HTML or XML. It allows developers to control the appearance of elements such as fonts, colors, spacing, positioning, and responsiveness. By separating content (HTML) from design (CSS), it enhances maintainability and flexibility. CSS supports features like animations, transitions, and media queries, enabling the creation of visually appealing and adaptive websites that work across different devices and screen sizes.