I wrote this up for a client, and realized I had given some version of this many times, so since I had it all written out I figured I’d post it here. I was trying to keep it somewhat high-level, as it’s generally targeted towards the non-developers.

Originally there was Flash. Flash was built as an animation tool and followed a timeline-based approach. When i say timeline-based, think of a filmstrip, and how it is made of a strip of individual, consecutive frames….that was the concept behind Flash, to make editing those long sequences of animation easier. If you open up an .fla file, you can clearly see that each layer is made up of consecutive frames…those consecutive frames are collectively referred to as a ‘timeline’.
Eventually, people figured out that they could build really cool stuff in flash, even full-blown applications that didn’t really have too much to do with traditional animation. Eventually the timeline metaphor, more than just becoming irrelevant, started getting in the way. People were building apps based more an an individual screen-based metaphor, and creating each of those screens on individual frames of the timeline could often be a chore.

So, in order to accommodate Flash application developers, as well as developers coming from other languages into Flash development that were getting scared off by Flash’s awkward development environment, the Flex framework and FlexBuilder were created. In general, the Flex framework is a code library of components which help to facilitate building applications that run within Flash Player. FlexBuilder is the Flex IDE (integrated development environment…big term for a code editor with some other development-related features thrown in) offered by Adobe, which really just a specialized build of Eclipse, which is an industry-standard open source IDE.

In addition to (or instead of) using FlexBuilder, the Flex SDK (software development kit) is open source, and it contains the Flex source code library and compiler, so you can develop Flex apps in any IDE. Setting up and using the SDK will provide the same results as using FlexBuilder, but requires more time and effort to set up…so you get all the tools for free, but you have to set them up yourself. That’s the tradeoff to using FlexBuilder where everything is prepackaged and set up for you already.

Flex application are written using a tag-based approach, so Flex apps are generally made up of text files containing a mix of markup that looks like html (this is called mxml), and Actionscript 3 code. When the flex application source code is compiled, the compiler turns the mxml tags are into normal Actionscript 3 code, and a .swf file is produced. That .swf file runs in Flash Player, the same as a .swf file produced from Flash.