The heart is the Flex-Server-Module for the J2EE-Server to run interactive Flash-like GUIs with AS.
It also contains an XML based interface called "Flex MarkUp Language" to define interactive GUIs.
This enables developers to create components in Flash format (Rich Internet Applications) including AS with standard, text-based development tools.
There is also a project called "Partridge" to add Flex development support to Eclipse.