Understanding the concept of prefix paths and overriding existing plugins will help you with your understanding of many components within the framework. Plugins are used in a variety of places:
Zend_Controller_Action: action helpers.
Zend_Form: elements, filters, validators, and decorators.
Zend_View: view helpers.
And several more places, besides. Learn the concepts early so you can leverage this important extension point in Zend Framework.
We'll note here that Zend_Controller_Front has a plugin system - but it does not adhere to any of the guidelines offerred in this tutorial. The plugins registered with the front controller must be instantiated directly and registered individually with it. The reason for this is that this system predates any other plugin system in the framework, and changes to it must be carefully weighed to ensure existing plugins written by developers continue to work with it.