FLEX

RIA developers are making applications that people didn't think were possible. With Flex, you can create full enterprise-scale Web applications that have as much interactivity as the best desktop applications. Flex enables some exciting possibilities

What is FLEX?
Adobe Flex is an application development platform that you can use to build Rich Internet Applications (RIAs). Flex applications are Web-based, but they provide immersive levels of interactivity and rich media experiences that make them seem more like computer desktop programs than traditional Web applications. Flex is the ideal technology to use when you want to create complex data visualization, performance dashboards, multimedia experiences, and countless other interactive applications. RIAs are raising the bar for Web applications, and Flex is leading the way
Flex is an application development framework and toolset that you can use to create RIAs. These applications are delivered over the Internet by using the Flash Player.

What an RIA (Rich Internet Application?
It describes a new model for application development that separated the back-end data services from a rich front-end client. One of the cornerstones of RIA development is the ability to asynchronously load data within the application. Simple HTML Web pages require a full page refresh to load new data. RIAs, on the other hand, load data asynchronously, which means they can load chunks of data without requiring page refreshes and they keep track of the application state in memory. Flex applications are stateful clients, which means they store data about the current state of the application, such as the content of a shopping cart, in memory in the client..
RIAs usually load data by using eXtensible Markup Language (XML). Asynchronously loading XML is an integral part of all RIA technologies (not only Flex). Version 4 of Flash, which was released in 1999, was the first version of Flash that let developers load external XML data into Flash applications

HISTORY
Flex 1 and 1.5, were expensive server-based products. A license for Flex 1.5 cost about $15,000, and you had to deploy a server application that would compile your Flex applications and serve them to the user. These initial versions of Flex were based on Flash Player 7 and ActionScript 2, and the code editor was based on the Macromedia Dreamweaver editor.
Flex 2 marked a dramatic shift in the product line. Flex 2 was no longer a server technology at all; instead, Flex was a completely clientside product. The cost dropped dramatically, and Adobe rewrote the entire Flex framework and the Integrated Development Environment (IDE) from the ground up. Flex 2 was based on Flash Player 9 and ActionScript 3, which brought serious performance gains
Flex 3 added additional functionality to Flex Builder, such as refactoring and enhanced styling support, as well as new data visualization components in the Flex framework. Flex 3 also marked the official open-source release of the Flex SDK and Flex compiler. For more on the open-source aspect of Flex, visit http://opensource.adobe.com.
Flex 4, codenamed Gumbo (coming soon)

FLEX Components
§ Languages: ActionScript 3 and MXML
You use a combination of ActionScript, a scripting language, and MXML, a markup language, to create Flex applications. MXML is similar to HTML, so if you have experience creating HTML Web pages, then you should be able to figure out MXML pretty easily.
§ Component framework: Flex SDK
The Flex SDK (also known as the Flex framework) is a set of user interface components, such as lists, buttons, and charts, that you use to build Flex applications. The Flex SDK (with the exception of the charting package) is free and open source.
§ Integrated Development Environment (IDE): Flex Builder
You use Flex Builder to edit your code, compile your applications, debug your code, and profile performance. Flex Builder is an integrated development environment (IDE) sold by Adobe.
§ Cross-browser runtime: Flash Player
You deploy Flex applications in a Web browser with the Flash Player plug-in. You can also deploy Flex applications as standalone desktop applications by using the Adobe Integrated Runtime (AIR).

Microsoft's Silverlight platform and Sun's JavaFX are two RIA development products aimed directly at taking on Flex
FLASH VS FLEX
§ The Flex framework: Flash has its own component set that has some of the same functionality as the Flex SDK, but it does not provide as many components and does not include charting components, layout containers, and other framework features that are very useful for developing large applications.
§ MXML: You can use MXML markup to create your Flex applications, but this markup language is not available in Flash. Flash does use the same ActionScript 3 scripting language, however.
§ A powerful Integrated Development Environment (IDE): Flex Builder was designed specifically to build applications, as opposed to the Flash Authoring tool, which was originally designed to create animations. You can use both tools to create RIAs, but Flex Builder has features like code-hinting, a powerful debugger, and a profiler that make it a more powerful development tool.
You can use Flash, rather than Flex, to create RIAs, but you have to work in the Flash Authoring environment, which means you don't get the benefits of Flex Builder (such as code hinting), and you can't use the MXML markup language. You may find the Flash Authoring tool really helpful if you're creating animated movies. It uses the timeline metaphor when you create animations and includes drawing tools. Flex Builder, on the other hand, is designed for application development, not animation creation. Flex Builder doesn't include any drawing tools, and it has no timeline.

Flex VS Ajax
Asynchronous JavaScript and XML (AJAX) is a technique that you can use to load data into HTML Web pages without refreshing the full page. Instead of sending a new HTML page to the user's Web browser for every change, AJAX applications send XML asynchronously and update relevant portions of the screen when needed. By using AJAX, you often end up with a much more responsive user interface and a more desktop-like application experience. Flex applications also asynchronously load XML data.

ActionScript VS JavaScript
Multimedia capabilities: Flash Player allows you to create a whole range of rich multimedia experiences that you simply can't achieve by using HTML and JavaScript. Flash has powerful graphics capabilities that can do complex drawing and image manipulation. In addition, Flash supports audio and video streaming, so many leading online video sites use it to play video on the Web.
Cross-browser support: You can be sure that any Flex application you develop will look and behave the same way in all browsers on all platforms. Web browsers all have their own quirks and idiosyncrasies when it comes to how they render HTML and even how they run JavaScript. When you develop AJAX applications, you need to test your application in multiple Web browsers to make sure your application is compatible with them all. However, because Flex applications rely on the Flash Player, you can be assured that your application will look the same, pixel for pixel, and behave the same across all browsers.

Flex VS SilverLight
Microsoft's competitive RIA technology is Silverlight, a browser plug-in, like Adobe's Flash Player. Just like you have to install the Flash Player plug-in to run Flex applications, you need the Silverlight plug-in to run Silverlight applications. Because Silverlight is fairly new, the Silverlight plug-in isn't nearly as common as Flash Player. Silverlight will likely become more widely used in the future, but right now, the Flash Player plug-in has a strong advantage because of the large number of computers on which it is installed.
The first release of Silverlight 1.0 included the browser plug-in and focused on streaming video on the Web. This release certainly competed with Flash Player because it focused on some of the media features of Flash, but it didn't really threaten Flex's position because Silverlight 1.0 didn't contain a set of user interface controls that you could use to build RIAs. Silverlight 1.0 provided all the low-level graphics capabilities but none of the application frame-work pieces.
In early 2008, Microsoft released the first beta version of Silverlight 2.0 (originally named Silverlight 1.1), which included a set of UI controls, such as a DataGrid, CheckBox, and Slider. These new controls make Silverlight a closer competitor with Flex. The competition between Flex and Silverlight has just begun, and it's too early to draw any firm conclusions about how serious a competitor Silverlight will become.
AIR (Adobe Integrated Runtime)
Adobe developed Adobe Integrated Runtime (AIR), previously code-named Apollo, to let you deploy Flex applications as computer desktop applications. By using AIR, you can create your own desktop applications that can run natively on Windows, Mac, and Linux operating systems. You can create AIR applications as Flex applications or AJAX applications, so if you know how to create Flex or AJAX applications, you can create desktop applications, too.
When deciding whether AIR is the right technology for your application, consider these three main features that AIR provides:
§ Local file-system access: One of the main reasons for moving from a Web-based Flex application to a desktop AIR application is the integrated local file-system access that AIR offers. If you build a Flex-based AIR application, you get a few extra tools that don't come in the normal Flex framework. These tools let you read and write to the user's local file system.
§ Integrated Web browser: The AIR runtime includes a built-in Web browser: the open-source WebKit browser. This Web browser allows you to load full HTML pages right into your application, something that you can't do in a Web-based Flex application. The AIR framework also lets you display PDF files within your application.
§ Embedded SQLite database: AIR applications can access an embedded database for offline database access. So, you can build applications that can connect to a server database (like a typical Flex application can) or to an offline database if your application is only occasionally connected to the Internet