A Cascading Style Sheet (CSS) is a stylesheet language used to describe the presentation of a document written in HTML or XML. It allows web developers to control the layout, colors, fonts, and overall visual appearance of a webpage, making it more appealing and user-friendly. CSS works by applying styles to specific elements on a webpage, which can be defined in separate files or embedded within the HTML. The "cascading" aspect refers to the way styles are prioritized and applied, allowing for flexibility and consistency across multiple pages of a website.