CSS (Cascading Style Sheets) is a style sheet language used for describing the look and formatting of a document written in HTML. It is used to control the appearance of web pages and to separate the content of a web page from its presentation.

CSS allows developers to apply styles to web pages in a consistent and modular way. It provides a set of rules that dictate how elements on a web page should be styled, such as their color, font, size, and layout. These rules can be applied to specific HTML elements or to groups of elements using class and ID selectors.

CSS is a powerful tool for customizing the appearance of web pages and can be used to create a wide range of visual effects, from simple text and color changes to complex layouts and animations. It can be written in plain text files or can be integrated into HTML files using style tags or linked to HTML files using link tags.

Overall, CSS is a key tool for web developers and is used to control the appearance and formatting of web pages. It is a widely used and well-established language that allows developers to create visually appealing and consistent web experiences.