The Microsoft Office Internet Free/Busy Service is a Web-based service provided by Microsoft that enables you to publish the blocks of time when you are free and when you are busy to a shared Internet location. This is convenient for people who don't normally have access to your Calendar but who do have Internet access. The service then works with Microsoft Outlook 2002 or later so that the times published to the service also display in a meeting request when someone is trying to schedule a meeting with you. Likewise, you can view their free/busy times and schedule meetings with them.
After joining the service, you change some settings in Outlook to enable it to work with the service. After you've done that, Outlook periodically publishes your free/busy times to the service. Anyone who isn't a member and who you haven't specifically authorized to view your free/busy times won't be able to access the information you publish to the service. Likewise, you won't be able to view others' free/busy times that are published there unless you are a member and they have authorized you to view their information.