Before we begin our exploration of the architecture of the Zend Framework (ZF), it is important to discuss how a typical MVC application is built. Examining and understanding the architecture of an MVC Web application allows you to make more contextually sound choices when building your application.
He starts with a look at a three-tier architecture (presentation, application, data) and compares that to the MVC used in the Zend Framework. He talks about how ZF is a hybrid component and infrastructure framework and the coupling that it provides between its components. He also has a few criticisms namely some performance issues from this style and the lack of a module to handle model/controller dependencies.