Everything in Flex occurs based on an event. This means that simply clicking a button or key to moving the mouse or receiving a response from a web service provides developers with the ability to trigger a custom event in their Flex applications. That’s a lot of control, and since Flex is not based on the request/response model of a standard web app, the app contains the functionality to perform those custom events without ever leaving the web page. This is because events are client-based—in other words, separated from the request/response model of a web page and do not require the page to refresh with every user interaction.
From Flash to Flex: Understanding Flex Events
Peachpit, Written by Kris Hadlock