Pug (formerly known as Jade) is a high-performance template engine heavily influenced by Haml and implemented with JavaScript for Node.js and browsers. It's designed to be a clean, whitespace-sensitive syntax for writing HTML. Pug files contain code that describes the structure and content of an HTML document. The Pug engine then processes this code and generates the corresponding HTML output. This allows developers to write HTML in a more concise and readable way, reducing boilerplate and improving maintainability. Pug supports features like variables, loops, conditionals, mixins (reusable code blocks), and includes (importing other Pug files). It's often used in web development frameworks like Express.js to dynamically generate HTML pages on the server-side. The main goal of Pug is to make HTML writing more efficient and less error-prone by enforcing a strict syntax and providing powerful templating features. It promotes code reusability and separation of concerns, leading to cleaner and more organized web projects. Pug is a popular choice for developers who value code readability and efficiency in HTML generation.