CSS stands for Cascading Style Sheets. It is a style sheet language used to describe the look and formatting of a document written in HTML, XHTML, or XML. CSS separates the presentation of a document from its structure, allowing developers to easily control the visual aspects of a website. CSS works by applying style rules to HTML elements. These rules define how elements should be displayed, such as their colors, fonts, sizes, positions, and more. The styles can be specified in an external CSS file that is linked to the HTML document or embedded directly within the HTML code using the `<style>` tags. CSS follows a cascading order, meaning that multiple styles can be applied to an element simultaneously. Styles can be inherited from parent elements or overridden by more specific styles defined for that element. This allows for efficient and flexible styling across an entire website. CSS offers a wide range of selectors that allow developers to target specific elements or groups of elements for styling. Selectors can be based on element types (e.g., `<h1>`), classes (e.g., `.header`), IDs (e.g., `#logo`), attributes (e.g., `[type="submit"]`), and more. CSS also provides various units of measurement for specifying sizes and positions, including pixels, percentages, ems, rems, and more. Additionally, CSS supports advanced features like animations, transitions, gradients, shadows, and responsive design techniques. With CSS frameworks like Bootstrap and Foundation available, developers can leverage pre-built styles and components to speed up development and ensure consistent designs across different devices. In summary, CSS is a powerful tool for controlling the visual appearance of web pages. It allows developers to create attractive and consistent designs while keeping the content separate from the presentation layer.