CSS (Cascading Style Sheets) is a language used to describe the visual appearance of web pages and user interfaces written in HTML (Hypertext Markup Language) or XML (Extensible Markup Language).

CSS is used to style and layout web pages, including elements such as fonts, colors, spacing, borders, and other visual attributes. It allows developers to separate the presentation of a web page from its content, making it easier to maintain and update.

CSS works by selecting HTML elements and applying styles to them. Styles can be defined within the HTML document itself, in an external CSS file, or within the HTML element’s “style” attribute.

CSS also supports the use of media queries, which allow developers to apply different styles based on the size and orientation of the screen or device. This is particularly useful for creating responsive web designs that adapt to different screen sizes.

Overall, CSS is an essential tool for web developers and designers, allowing them to create visually appealing and well-organized web pages that are easy to maintain and update.