Yii is a high-performance, component-based PHP framework designed for developing web applications quickly and efficiently. It follows the Model-View-Controller (MVC) architectural pattern, which helps separate the application logic from the user interface, making it easier to manage and scale applications. One of the key features of Yii is its extensive set of built-in tools and libraries, which streamline common tasks such as database access, form validation, and user authentication. This framework is particularly well-suited for developing large-scale applications due to its speed and flexibility, allowing developers to create robust and maintainable code.