Adobe Flex is a cross-platform, cross-browser platform for creating Rich Internet Applications. It provides a modern, standards-based language and programming model that supports common design patterns. MXML, a declarative XML-based language, is used to describe UI layout and behaviors, and ActionScript™ 3, a powerful object-oriented programming language, is used to create client logic. Flex also includes a rich component library with more than 100 proven, extensible UI components for creating rich Internet applications (RIAs), as well as an interactive Flex application debugger.
Cross-Platform, or also known as Multi-Platform is any application that is able to run on all operating systems. These systems include Windows, Mac, Linux, etc. Adobe Flex exports its application using the Adobe Flash Player 10+. Any computer that runs this simple software, will be able to run any Flex application.
Cross-Browser is a term to describe how an application can run in any browser regardless of version. The only requirement that a Flex application needs is Adobe Flash Player 10+, and they're good.
The new Adobe® AIR™ runtime lets developers use Adobe Flex® technology to build rich Internet applications (RIAs) that deploy to the desktop. AIR applications run across operating systems and are easily delivered using a single installer file. With AIR, Flex developers can use their existing skills, tools, and code to build highly engaging, visually rich applications that combine the power of local resources and data with the reach of the web.
Benefits of Adobe AIR for developers using Flex include: