An application event is something that occurs which might be of interest to other bits of code. A standard event in most GUI applications would be a "click" event, but the sky's the limit and what events you define is really up to you. [...] The details can be provided by application-specific modules allowing you to keep individual requirements separate from your reusable code. Events allow you to attach a potentially unlimited amount of functionality without changing your core modules and components.
In his example, he shows how to create an event handler that is triggered when the user registers on the site. This event (CEvent) is then registered with the system and is attached via a call in the controller's "init" method.