Cascading Style Sheets (CSS) is a style sheet language used to describe the appearance and formatting of a document written in HTML. CSS is used to control the style of a web document in a simple and user-friendly way.

CSS allows you to specify how HTML elements should be displayed. You can control things like the font, colour, size and spacing of elements on a page. You can also specify how elements should be positioned on the page or how they should react to user interactions.

CSS is a powerful tool to separate the content of a web page (written in HTML) from its presentation. This separation of content and presentation makes web pages easier to maintain and update, as you can change the presentation of all pages on a site by changing a single file.

CSS is used in conjunction with HTML to create dynamic and interactive web experiences for users. It is an essential part of modern web design and essential for creating visually appealing and user-friendly websites.