What is Flex?
Flex is the way to make Rich Internet Applications quickly and easily. It’s a framework for creating RIAs that run with widely deployed Flash Player. At its heart it uses two languages namely MXML, a markup language based on Extensive Markup Language (XML) and ActionScript an EMCAScript based native scripting language of Flash platform.
Why Flex?
Quick, Easy, Well Supported: Flex gives you interactivity in a robust development environment. The Flash Platform gives you more reach and more tools than any other technology out there. It really is the next generation of web applications and Flex has made it easy to jump in and take advantage.
Flex is nothing but Flash: Just like Flash movies Flex is deployed as SWF files to be played in Flash run-time environments like Flash Player in browser, Adobe Integrated Runtime (AIR) and Flash Lite (mobile Flash player).
Asv3's Blog 