Macromedia Flex is an application server initially released in March of 2004. In short it is a J2EE application that compiles on the fly Flex mark up language (MXML) and Actionscript into Flash applications.
The goal of Flex is to allow Web application developers to easily build Rich Internet Applications, otherwise known as RIAs. In a multi-tiered model, Flex applications serve as the Presentation Tier. These applications were already possible using Macromedia Flash MX 2004; however for non Flash developers many found it challenging to develop in an animation based environment.