Understanding Messages and Events
Macintosh applications perform tasks in response to events. Users originate events with the keyboard and mouse or touchpad, and applications respond to the events by performing tasks. Similarly, an application can make other applications perform tasks by sending messages about events.
The events that applications send to each other in messages are called Apple events. AppleScript makes applications perform tasks by sending them Apple events.
When an application receives a message about an Apple event, the application takes a particular action based on the specific event. This action can be anything from performing a menu command to taking some data, manipulating it, and returning the result to the source of the Apple event message.
For example, when you choose 
⇒Shut Down or ⇒Restart, OS X sends an Apple event message to every open application saying a Quit event occurred. For this reason, applications quit automatically when you choose ⇒Shut Down or ⇒Restart.
When you drop document icons on an application icon, the Finder sends a message to OS X saying ...
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