Adding to the other comments, I find it easiest to think of Flex as a Flash-version of HTML, CSS, JavaScript, and pick-your-favorite-JS-libraries such as jQuery and Sencha. Instead of HTML, Flex has MXML. Instead of JS, it has ActionScript. (Though both come from the same language standard.) CSS works, and they bundle a good set of default themes and widgets. I think it's a really nice piece of work, especially when they put out Flex 4, it's a joy to work in compared to raw HTML+JS. The only downsides are Adobe, the flash runtime, and the intellectual heritage of Java sometimes getting in the way.