CSS (Cascading Style Sheets) is a stylesheet language used to describe the look and formatting of a document written in HTML. It allows to separate the content and presentation of a website, making it easier to maintain and update the visual appearance of a website. CSS provides a range of styling features, including text and font styling, colors and backgrounds, layout and positioning, and various effects and animations. These features can be used to create a wide range of styles, from simple and minimalistic to complex and visually rich designs.