One of the benefits of a Flex application is that the application can smoothly transition from state to state without having to fetch a new page from the server and refresh the browser. By avoiding the constant refreshing of pages, the end-user’s experience is more fluid and continuous. In addition, the load on the server is greatly reduced because it need only return the application once, rather than a new page every time the user changes views.

However one of the advantages of a browser’s page-oriented model is that an application’s navigational state is usually clearly coupled to a URL. Thus, when the state of an application changes, the user can usually do the following:

* Bookmark the URL to get back to that state in the application

How it works: Deep linking relies on communication between the browser and the Flex application. The communication is bidirectional: if a change occurs in the application, the browser must be notified, and if a change in the browser occurs, then the application must be notified. This communication is handled by the BrowserManager class. This class uses methods in the HTML wrapper’s JavaScript to handle events, update the browser’s address bar, and call other methods. Another class, URLUtil, is provided to make it easier to parse the URL as you read it in your Flex application and write it back to the browser.

more: http://livedocs.adobe.com/flex/3/html/help.html?content=deep_linking_2.html