Everything from YouTube videos and Flickr slideshows to addictive Web-based video games runs on Adobe Flash. Flash is so popular because it's platform independent (i.e., it works on any operating system and many mobile devices) and handles media and graphics with aplomb. But its applications can't stand on their own two feet. Instead, they require a host application to run--most often, your Web browser. That's where Adobe AIR comes in.
AIR is a relatively new platform, introduced to the public by Adobe in 2008, that creates standalone applications that run from your desktop just like any other application. AIR applications are developed in very much the same way Flash applications are, providing developers with a new environment to create powerful Web applications that run as standard desktop application--no browser necessary.
Despite AIR's impressive ability to blur the line between your desktop the Web, don't think of AIR applications as Web-only; as you'll see, AIR apps can run both when you're online and offline, and some of them have nothing to do with the Web. The real reason you should install AIR: It provides a framework and powerful toolset that allows developers to quickly create and deploy applications to any platform, and as such, a lot of great software has already been built using AIR.
To get started using AIR applications, all you need to do is download and install AIR from Adobe. Once you've done so, downloading and opening an AIR application on your desktop will trigger the standard AIR installer (think of these files like traditional .msi or .exe setup applications, except AIR handles the installations).
Note: This link takes you to the vendor's site, where you can download the latest version of the software.