Adobe have taken the run-anyware idea of web browser and web-server and created a desktop application framework. This means you can build a "website" that runs without a server.
There are a few nice things with this approach. The main one is that it makes it possible to do things locally that a website cannot do, like read and write files, or create native windows. And because the "browser" it runs in is a known quantity, you can take advantage of that with WebKit-specific extensions. Or you can just build it in Flash. Or combine the two as you need.
Adobe have also leveraged the cross-platform quality: both key pieces of AIR (Flash and WebKit) are already available on Windows, MacOS and Linux, so it was not that much of a stretch to make the whole of AIR cross-platform. This gives a really neat effect: the same .air file should install on any AIR install. And will run the same.
