Quoted;
If you haven't already tried out the big form at the top of the page, what it does is turn a website - any website (we hope) - into a desktop application. Instantly. And that's it!

This can be especially useful for web apps, such as Gmail, Google Reader, Bloglines, Facebook, Netvibes, etc - any web site which isn't really a bunch of pages, but is really a software application. They don't really belong mixed in with your browser tabs - you're not browsing them, you're using them! Once you've AIRified them, they move out of the browser and onto your desktop.