With the launch of Flex 3 and Adobe AIR, there is a lot of hype these days around Flex.

Flex is a  framework that lets us write code that feels more like coding a desktop application—except it runs inside the Flash player! Because it targets the Flash player, we can build new Rich Internet Applications (RIAs) without worrying about browser compatibility nonsense, JavaScript, CSS, and so on.
Because Flex 3 targets one platform (Flash 9), we don’t have to worry about platform compatibility issues. Flex achieves what previous technologies failed miserably in attempting: applications that feel like desktop applications, but which run inside any modern web browser on Windows and Mac.

We can use Flex 3 to build RIAs today that look and feel more like Web 3.0 than many of the “me too [point oh]” sites you see copying 37signals and each other.
 

In Flex , we write code in MXML (XML files with an .mxml extension; M for Macromedia) and ActionScript (text files with a .as extension) files and compile them into a SWF file (that is, a Flash movie), which runs in the Flash player. This SWF is referenced by an HTML file, so that when a user with a modern web browser loads the HTML file, it plays the Flash movie (prompting the user to download Flash 9 if it’s not present). The SWF contained in the web page can interact with the web page it’s contained in and with the server it was sent from.

Even if you’ve never created a Flash movie in your life, don’t consider yourself a designer, and wouldn’t recognize the Timeline if you tripped over it, you can use Flex to create attractive applications that run in the Flash player.

Flex is very productive. With Flex we habe the best ussability for web applications.