AIR adds a number of desktop-specific classes and components to Adobe's base Flash and Flex classes. AIR 1.5 applications can update themselves, interact with the system clipboard, use the file system, use native windows and menus, use a local SQL database, and store encrypted data. AIR also supplies a number of capabilities to the desktop environment for which Flash and Flex applications normally rely on the browser -- for example, HTML rendering, HTTP handling, and network detection.
Publicly available Adobe AIR applications at the AIR Showcase range from simple desktop widgets to full-blown applications. AIR is supported for development and runtime on Windows, Mac OS X, and Linux.
I am more of a Flex and AJAX developer than a Flash designer. I found the AIR extensions to Flex fairly straightforward to learn and use from Flex Builder, although I would have been happier if the AIR documentation were included in the Flex Builder help file instead of only being available online.
AIR runtime, AIR SDK, Flex 3 SDK, and Aptana Studio are free. Pricing for Aptana Studio Pro is $199, $699 for Adobe Flash CS4 Professional, $249 for Adobe Flex Builder 3 Standard, $699 for Adobe Flex Builder 3 Professional, and $399 for Adobe Dreamweaver CS4.
Nitro is the cross-platform desktop extension to Curl. Currently in its second beta-test release, Nitro adds an applet installer, desktop controls and a client-side SQLite database to Curl's excellent RIA capabilities. Nitro applications take advantage of Curl's high-performance, skin-able user interface and built-in security sandbox. Even without Nitro, detached Curl applets can be used for desktop applications: The Curl IDE and help system are good examples of that.
Curl may not be as familiar to you as the competing products from Adobe (AIR), Google (Gears), and Microsoft (Silverlight), but you shouldn't let that deter you from trying it. It has CPU-bound runtime performance roughly 10 times that of Adobe AIR, according to a test of JPEG encoding conducted by Curl in May. (The test was pooh-poohed as "irrelevant" by Adobe, but not refuted.)