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 is the product of Adobe and it is free open source framework for creating and maintaining web applications.Flex applications can be written using Adobe Flex Builder or by using the freely available Flex compiler from Adobe. The work on the Flex SDK has been focused around three primary themes:

• Design in Mind: it provides a framework that helps you to support a new degree of lucidity, clarity, easily enabled with tools without many efforts.
• Developer Productivity: enhancing compiler functioning, performance and adding productivity to language features like data binding.
• Framework Evolution: by incorporating the new characteristics in flash the common use-case features are added

Flex is a way to develop Flash applications by programming. It includes a declarative XML language called MXML for laying out user interfaces, and a programming language called ActionScript with extra features like optional static type checking.
Flex applications compile directly into SWFs (Flash binaries), which are then Just-In-Time (JIT) compiled by the Flash runtime, for extra speed.

Action Script

ActionScript is a single language that works across all platforms. Since it is based on ECMAScript, your JavaScript knowledge is not lost. All MXML components are actually written in ActionScript, which you use if you want to write your own components.
You can download the free command-line Flex compiler to create static SWFs, and you can deliver these from your web site without paying any fees. The compiler, framework, and debugger are all free, so there’s no reason to avoid using Flex.
You can buy the Flex Builder IDE to help you create Flex apps. It contains the features like auto completion, context help, debugging, and even a GUI layout tool. The layout tool can give you a quick start when you’re beginning a design.

Rich user experience

Flex allows web application developers to efficiently create highly interactive, expressive interfaces for web and desktop applications. The applications built with Flex can reach more users, improve productivity and increase profits.

Cross-platform, accessible applications

Flex applications leverage the Adobe® Flash® Player 9 runtime. Installed on over 98% of Internet connected computers, Flash Player uniquely provides a consistent, accessible user experience across browsers and platforms. It is an enterprise-class client runtime with advanced vector graphics capable of handling the most demanding, data-intensive applications while performing at desktop application speeds.

Adobe AIR integration

The new Adobe AIR™ client runtime enables rich Internet applications (RIAs) to run on the desktop, creating new opportunities for more engaging, high-performance online/offline applications. The Flex framework provides native support for the new AIR APIs, and Adobe Flex Builder 3 software provides all the tools necessary to build, debug, package, and sign applications built on Adobe AIR.

Developer productivity

You have more than100 rich, prebuilt application components. Use the powerful Eclipse based Flex Builder development environment to further accelerate development, debugging, and testing of web and desktop RIAs.

Adobe Creative Suite 3 integration

The combination of Flex and Adobe Creative Suite® 3 software provides smooth, consistent workflows using best-in-class tools. Use Adobe Flash, Fireworks®, Illustrator®, and Photoshop® to create assets in native Flex formats. Use Flex Builder to easily import these assets into the interface of an RIA.

Ease of use

Get started quickly with an extensive library of components, skins, containers, and application services. Use wizards to connect to existing web services or generate database connection code in Adobe ColdFusion®, PHP, ASP.NET, and Java™.

Open source, standards-based framework

Flex 3 is available as open source software through the Open source flex SDK project. Flex provides a modern, standards-based language and programming model. You can extend and enhance the open source framework to suit your needs.

Advanced data services

You can easily connect Flex and AIR applications to back-end, distributed data and Java server infrastructures to deliver responsive, real-time, data-driven user experiences for the browser or the desktop.

Enhancements to existing projects

Add by embedding micro-applications or components in existing websites

Scalability

Flex is designed to scale from simple components in websites to full-fledged browser-based or desktop applications. With its prebuilt component library, Flex provides a broad solution for creating RIAs of all kinds.

Flex as a DSL for Graphics

One of the most appealing things about Flex is that it was created with the idea of UI first. In a very real sense, it’s a domain-specific language (DSL) for graphics, multimedia, and UIs, whereas most other solutions have been languages with UI libraries tacked on afterwards.
Because of this design goal, Flex and Flash provide a complete, unlimited, flexible tool to build user experiences. From the standpoint of a programmer’s time investment, you can learn a single language for building UIs without worrying about running into issues like:

• Installation problems
• Constraints on what you can create
• Sudden steep climbs in the learning curve

There are plenty of fancy components that you can just drop in and use—the Flex Framework (free download) comes with over 100 components. There’s an active marketplace for component creators, both open-source and pay ware. One such library comes from Adobe: the Flex Charting Components (within few hundred dollars), but there are also competing charting components.

Flex Data Services

It’s a publish/subscribe API for data management. Flex Data Services automatically performs caching and updating between client and server, to produce an optimal experience without forcing you to write extra code. You can use Flex Data Services on a single CPU for free; if your application requires multiple CPUs you’re considered an enterprise and the licensing fee kicks in.

Flex on the Desktop If your dream is to be able to learn a single GUI system in depth, Flex the right tool.
A Flex UI is not limited to communicating with a server. It can also communicate with a local application. Thus, you can create an application in any language you prefer.

Adobe Flex Builder 3

It is a highly productive Eclipse based development tool enabling intelligent coding, interactive step-through debugging, and visual design of the user interface layout, appearance, and behavior of rich Internet applications (RIAs).

Conclusion

Flex is a free, open source framework for building highly interactive web applications that works consistently on all major browsers and operating systems. It provides a modern, standards-based language and programming model that supports common design patterns. MXML, a declarative XML-based language, is used to describe UI layout and behaviors, and ActionScript 3, a powerful object-oriented programming language, is used to create client logic. Flex also contains a rich component library with more than 100 extensible UI components for creating rich Internet applications (RIAs), as well as an interactive Flex application debugger.