In 2005 Adobe, the makers of the graphic design applications Photoshop and Illustrator, bought a company called Macromedia. Flash was Macromedia's baby. It's an application used to create animation for the web. Flex is the software development kit (SDK) used by Flash to combine functionality with animation. ActionScript is the code language that produces this functionality.

In terms of output Flash/Flex/Actionscript is akin to JavaScript, though the platforms are completely different when it comes to construction. You can build an entire website with Flash or specific components like video and music players. Flash is a specialized discipline and you rarely find developers who know Flash and other database-oriented code languages like PHP. It's also time-consuming to modify and requires a pro to make significant changes.

Professional Flash websites look super-cool because they are animated but Flash isn't the best option for small budget projects or sites that require frequent updates. If you're building a website that needs to look swank and the content will never change, Flash is an option to consider. If you're intending to update your website and you're not sitting on bags of money, you're better off with a content management website. You can incorporate Flash elements into your CMS site but I suggest you only use it for display and not functionality.