Flash ActionScript 3 Event Basic Concept
An event is simply a runtime occurrence that will trigger a response in the program. The most common example of events are Mouse Click and Keyboard Input.
Not all events will be responsed. For example, if you click on the top flash banner on this website, no action will be responsed.
Actually when an event need to be responsed, it need to ask whom to handle and how to react it. This can easily be explained with the following diagram:
Therefore Alex listen somebody called "Alex", he will response according to the given instruction. This is shown in the following diagram: