One of Outlook's most useful features is the capability of providing you with free/busy information for everyone in your organization. It appears to the user to be something that "just works." If, however, you've ever managed Exchange 5.5, 2000 or 2003 and had public folder corruption or just problems with free/busy data in general, you know that this feature is far more complicated than it first appears. I'll talk very briefly about how free/busy information is handled with Outlook 2003 and Exchange 2003 and will then explain how Microsoft has improved free/busy handling in Outlook 2007 and Exchange 2007 with the new Availability Service.
In Exchange 2000 and 2003, and with clients older than Outlook 2007, Microsoft has provided a mechanism by which a special public folder is used to store users' free/busy information. This folder is named SCHEDULE+FREE BUSY, and has a number of subfolders, each corresponding to your separate administrative groups. As users populate calendars, Exchange also updates the appropriate subfolder with the same information. If you would like to see this special system public folder, open the Exchange System Manager and go to Folders > Public Folders. Right-click Public Folders, and, from the resulting shortcut menu, choose View System Folders. As you have probably guessed by now, storing free/busy in public folders means that, while information is eventually updated, there can be a lag between the time a user updates his calendar and when the free/busy information becomes available. Further, in large Exchange organizations, this information must be replicated all over the place.
Now, go to Google and type "free/busy problem" and wait for the results to greet you. I won't go into the specifics about why this happens since there are way too many reasons to list. But I can tell you that, speaking from experience, correcting free/busy problems can be incredibly delicate and time consuming and you find out very quickly how much users like the feature!
Enter the Exchange 2007 Availability Service.
The Availability Service is installed as a part of the Client Access Server role in Exchange 2007. Instead of accessing a public folder to pull free/busy information for attendees, the Availability Service just polls—in real time—the mailboxes for each of the attendees you list and provides you with free/busy information. Besides cutting out the "middle man" of public folders, the Availability Service just about guarantees that calendar information is current.
Even better, the Availability Service can cross forest boundaries. With older versions of Exchange, crossing forest boundaries was difficult, but the Availability Service makes this very easy.
It's not all roses, though. Until you deploy Outlook 2007, you can't take advantage of the Availability Service. However, in order to support mixed Outlook 2003/2007 organizations, the Availability Service is still able to pull free/busy information from the system public folder, if you chose to install one when you installed Exchange 2007. But if you have users making heavy use of Outlook Web Access 2007, this web service uses the Availability API for all of its free/busy work, so those users will gain the feature's benefits.
Eventually, there will be a day when the SCHEDULE+FREE BUSY folder is a thing of the distant and horrible past and we can all rejoice!