Flex is a free, open source framework for construction highly interactive, expressive web applications that arrange consistently on all major browsers, desktops, and operating systems.

Flex includes a rich part library with over 100 proven, extensible User Interface parts for generating rich Web applications, as well as an interactive Flex application debugger.

The Flex also provides a contemporary, standards-based language & programming model that supports common design patterns.
RIAs created with Flex can run in the browser using Adobe Flash Player program or on the desktop on Adobe AIR, the cross-operating method runtime.

Flex applications can now access local information and method resources on the desktop.

Both Flash Player & Adobe AIR are available as a free download on Adobe.com.

A Flex application at it’s basic level is a Flash .swf file embedded in a generated HTML file. The HTML that Flex spits out has the necessary JavaScript code in order to notice whether the user has the Flash Player installed.

The environment of Flex does not use a Timeline. Flex has been developed more for programmers and less for designers. Flex is a convoluted term for definite. As the history of the product has evolved it’s been a j2ee server, an as framework, a compiler and IDE among other things.

The term flex usually refers to a way of developing flash platform application using the parts in the open source framework developed by adobe.

Flex applications are written as a combination of xml markup and actionscript much in the same vein that html applications are written with JS.

The huge difference is that the output of all of your mxml and actionscript files is a single swf that can be embedded in a web page & exhibited to the world.