CSS (Cascading Style Sheets) is a fundamental web technology that defines the presentation and layout of HTML documents. Utilized in conjunction with HTML, CSS enables web developers to control the visual aspects of a website, including colors, fonts, spacing, and overall styling. It follows a cascading hierarchy, allowing styles to be applied consistently across multiple pages. CSS promotes separation of content and design, enhancing the maintainability and flexibility of web development projects. With its ability to create responsive and visually appealing designs, CSS plays a crucial role in shaping the modern web user experience.