Stylus is a preprocessor scripting language that is compiled into Cascading Style Sheets (CSS). It is designed to be a concise, flexible, and expressive way to write CSS. Stylus allows developers to use variables, functions, mixins, and other programming constructs to create reusable and maintainable stylesheets. It supports both indentation-based syntax (similar to Python) and traditional CSS syntax. Stylus files are typically used in web development projects to streamline the CSS authoring process, reduce code duplication, and improve the overall organization of stylesheets. The resulting CSS files are then linked to HTML documents to style the web page elements. Stylus offers features like automatic vendor prefixing, color manipulation, and mathematical operations, making it a powerful tool for front-end developers.