This is an excellent video tutorial on how to build an Adobe Flex web application (RIA). As you can see Adobe Flex and Air are the same beast, the difference being where they are aimed at, the desktop or browser flash player.
Flex is a highly productive, free open source framework for building and maintaining expressive web applications that deploy consistently on all major browsers, desktops, and operating systems. While Flex applications can be built using only the free open source framework, developers can use Adobe® Flex® Builder™ software to dramatically accelerate development.
http://www.adobe.com/products/flex/media/flexapp/
