CSS (Cascading Style Sheets) is a stylesheet language used to describe the presentation of a document written in HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS describes how elements should be rendered on screen, on paper, in speech, or on other media. CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript. It allows developers to separate the content of a web page (HTML) from its visual presentation (CSS). This separation makes websites easier to maintain and update, as changes to the style can be made in a single CSS file, rather than having to modify every HTML page. CSS uses a simple syntax consisting of selectors, properties, and values. Selectors target specific HTML elements, properties define the visual characteristics (e.g., color, font, size), and values specify the settings for those properties. CSS files are typically linked to HTML documents using the `<link> tag, or the styles can be embedded directly within the HTML using the <style>` tag. External CSS files are generally preferred for larger websites to improve organization and maintainability.