It's similar to HTML. You type out an XML-like file, and upload it to your server. The differences: you need a server with Flex installed; the source code is compiled into a Macromedia Flash file, and the server makes a page that loads that Flash file; you need to make a whole bunch of configuration files to use your Flex "application"; and in general, it's expensive and designed for large companies who have their own servers.
Bottom line: So much to learn, so much to pay, the average web developer or website administrator won't like it. If you want to try it, download he installer from Macromedia's website, and install it (you don't need admin access on Windows 2000/XP). Otherwise, forget it exists.
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