Flex is a complete, powerful application development solution for creating and delivering cross platform rich Internet applications (RIAs) within the enterprise and across the web. It enables the creation of expressive and interactive web applications that can reach virtually anyone on any platform.
Flex is a client-side technology that is rendered by Flash Player and requires integration with a server-side technology (like php, Java, ASP.net etc.) to provide it with real-time data.
Flex is a highly productive, open source application framework for building and maintaining expressive web applications that deploy consistently on all major browsers, desktops, and devices.