A CSS (Cascading Style Sheet) file is a text document that contains style rules for controlling the presentation of HTML or XML documents. It defines how elements should be displayed, including aspects like font, color, spacing, layout, and responsiveness. CSS separates content from presentation, allowing for a more organized and maintainable website structure. By using CSS, developers can apply consistent styling across multiple web pages, making it easier to update and modify the look and feel of a website. CSS files are linked to HTML documents using the `<link> tag, or the styles can be embedded directly within the HTML using the <style> tag, although external CSS files are generally preferred for better organization and reusability. CSS uses a selector-based syntax to target specific HTML elements and apply style rules to them. These rules consist of properties and values, such as color: blue; or font-size: 16px;`. CSS plays a crucial role in web development, enabling developers to create visually appealing and user-friendly websites.