Adobe Flex is a software development kit (SDK) 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 release in March 2004 by Macromedia included an SDK, an Integrated development environment (IDE), and a Java EE integration application known as Flex Data Services. Since Adobe purchased 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.
source : wikipedia