A .tpl file is a template file used by the Smarty templating engine, a popular PHP web template system. These files contain a mix of HTML, CSS, JavaScript, and Smarty's own template language syntax. The Smarty syntax allows developers to embed PHP variables and logic directly into the HTML structure, making it easier to separate the presentation layer from the application logic. This separation improves code maintainability, reusability, and collaboration between designers and developers. The .tpl files are processed by the Smarty engine, which replaces the Smarty tags with the corresponding PHP values and logic results, ultimately generating the final HTML output that is sent to the user's browser. Smarty provides features like caching, template inheritance, and plugin support, making it a powerful tool for building dynamic web applications. Using .tpl files helps to enforce a Model-View-Controller (MVC) architecture, promoting cleaner and more organized code.