Yii is a high-performance PHP framework designed for building modern web applications. Known for its simplicity and extensibility, Yii offers robust features like caching, authentication, and role-based access control. Its MVC architecture enables clean and organized code, while the framework's tools, like Gii, accelerate development. Yii is particularly suitable for developing scalable applications, including e-commerce platforms and CRM systems. Its active community and extensive documentation make it a reliable choice for PHP developers.