CSS (Cascading Style Sheets) is a fundamental technology used in web development to control the visual presentation of HTML documents. It provides a flexible and efficient way to define the layout, colors, fonts, and other design aspects of a website. With CSS, web designers can separate the content from its presentation, enabling consistent styling across multiple web pages. CSS utilizes selectors to target specific HTML elements and applies styles to them, allowing for precise control over the appearance of the webpage. It supports various styling techniques such as box model, flexbox, and grid layout, offering responsive designs that adapt to different screen sizes and devices. CSS plays a crucial role in creating visually appealing and user-friendly websites, enhancing the overall user experience.