CSS (Cascading Style Sheets) is a stylesheet language used to describe the presentation of HTML or XML documents. It controls the layout, colors, fonts, and overall visual appearance of web pages. CSS allows for the separation of content (HTML) from design, making websites more flexible and easier to maintain.

Key concepts in CSS include:

• Selectors: Specify which HTML elements to style (e.g., p, .class, #id).
• Properties: Define the styles (e.g., color, font-size, margin).
• Values: Assigned to properties (e.g., color: blue;, font-size: 16px;).

CSS can be included directly in HTML files (inline, internal) or in separate external files, making it reusable across multiple pages. Modern CSS includes advanced features like animations, transitions, and responsive design techniques using media queries.