CSS, or Cascading Style Sheets, is a stylesheet language used to describe the presentation of a document written in HTML or XML. It enables web developers to separate content from design, allowing for greater flexibility and control over the layout, colors, fonts, and overall aesthetics of a webpage. By using CSS, developers can create visually appealing websites that are responsive and adaptable to different screen sizes. CSS also supports various features like animations, transitions, and grid layouts, making it an essential tool for modern web design.