What is Flex?

Adobe Flex is a software development kit released by Adobe Systems for the development and deployment of cross-platform rich Internet applications based on the Adobe Flash platform. Flex applications can be written using Adobe Flash Builder or by using the freely available Flex compiler from Adobe.
The initial release in March 2004 by Macromedia included a software development kit, an IDE, and a Java EE integration application known as Flex Data Services.
Since Adobe acquired Macromedia in 2005, subsequent releases of Flex no longer require a license for Flex Data Services, which has become a separate product rebranded as LiveCycle Data Services. An alternative to Adobe LiveCycle Data Services is BlazeDS, an open-source project that started with code contributed in 2007 by Adobe.
In February 2008, Adobe released the Flex 3 SDK under the open source Mozilla Public License and so Flex applications can be developed using any standard IDE,
for example Eclipse. There is also a commercial and proprietary IDE called Adobe Flash builder.