Cascading Style Sheets is a style sheet language used for describing the presentation of a document written in a markup language such as HTML. CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.

A style sheet language used for formatting and positioning the presentation of your pages markup language, most often HTML.

CSS is one of the three major cornerstone technologies of the internet, alongside HTML and JavaScript

Need more details? Read the Mozilla Developers resource on CSS here.