Jade (now known as Pug) is a high-performance template engine heavily influenced by Haml and implemented with JavaScript for Node.js and browsers. It is used to generate HTML markup in a concise and readable format. Jade/Pug emphasizes code readability and reduces boilerplate by using indentation instead of closing tags, similar to Python. This makes the templates cleaner and easier to maintain. It supports features like variables, loops, conditionals, mixins (reusable code blocks), and includes (importing other templates). Jade/Pug templates are compiled into HTML, making it a server-side templating language. It is commonly used in web development frameworks like Express.js to dynamically generate HTML pages. The file extension '.jade' is used to store the source code of these templates. While the project is now named Pug, many existing projects still use the '.jade' extension. Using Jade/Pug allows developers to write more efficient and maintainable HTML code, reducing redundancy and improving overall development speed.