Pug is a high-performance, concise template engine heavily influenced by Haml and implemented with JavaScript for Node.js and browsers. It transforms Pug templates into HTML, offering features like dynamic content insertion, conditionals, loops, and reusable mixins, making it efficient for building complex user interfaces. Its readable syntax promotes cleaner codebase and rapid development by abstracting HTML verbosity. It's widely used for server-side rendering and client-side rendering alike.