From the Macromedia Flex website (http://www.macromedia.com/software/flex/):
The presentation-tier solution for delivering enterprise Rich Internet Applications
The Flex presentation server delivers a standards-based programming methodology and runtime services for developing and deploying the presentation tier of applications that combine the richness of the desktop with the reach of the web – Rich Internet Applications.
Flex allows dynamic creation of Macromedia Flash movies through XML code, creating a Flash presentation layer that can process and display your dynamic data without constant page redraws and requests to the application server. Instead of developing a Flash interface to your data in Macromedia Flash, requiring a thorough understanding of movie timelines, symbols, and animation concepts, Flex allows you to create your Flash movie in code, either manually in your favored text editor, or visually with the assistance of Flex Builder.
This HOWTO is intended as a complete, step-by-step guide to installing and setting up Apache Tomcat servlet container and the Macromedia Flex presentation tier application server.