I’ve had a couple questions come at me lately about what is Flash Builder and how does it relate to Flex now.
This is nothing too technical, just a simple explanation.
Flash builder is just Adobe’s Tool to develop Flex applications. It was formerly Flex Builder, but Adobe is attempting to unify the Flash name and branding across its tools so they changed the name. It is built on Eclipse which is a general IDE, but again, the focus is that it is simply the tool, and the Flex framework is still Flex.
The Flex framework primarily is based on MXML, a mark up language developed by Adobe. You can also use AS 3.0 to supplement the MXML. And yes, you can even tie it into other frameworks and languages such as Rails, Cold Fusion, PHP and a quite a bit more, especially if you need some server side tasks.
Flex compiles to a swf, similar to Flash, and utilizes the Flash Player in order to render it, or you can wrap it into an AIR desktop application.
An analogy is like HTML. It is a mark up language based framework for web pages. You can use a variety of tools to construct an HTML page(s), such as adobe Dreamweaver, Esspresso, Taco Edit, TextEdit, and even Notepad. You can also supplement it with Javascript. Of course, you can also tie it into the other frameworks and languages such as Rails, Cold Fusion, PHP, ASP.NET, and more. You can also use Flex and Flash to create components to embed into your web page such as video players and widgets. You can also wrap a web page into an AIR desktop application.