By default, others within your organization can only see whether you are free or busy for a particular timeslot. In theory, people should check whether you are available before scheduling an appointment with you, although nobody seems to do that for me. Hooray for double and triple-booking!
That said, it is possible to let others see more information about your calendar appointments. This can be useful in allowing a co-worker to see details about your calendar so that they do not schedule over an important meeting with a VIP. This can also be dangerous in a world where personal appointments often make their way onto the work calendar. Do you really want your co-workers to see the following appointment?
I just took a look at who had permissions to my calendar, and it was a wide array of folks that I have worked with over the last 3-4 years, and very few of them have a business need to see the actual contents of my calendar anymore. So… here are a few places you can check your permissions in Outlook 2010 (the process should be relatively similar in Outlook 2007, but I only run the latest and greatest ;)
When sharing your calendar, you have the option of choosing how much information is shared with the recipient.
Keep in mind that whatever permissions you give will be given until they are revoked (and if you don’t check regularly, people could have permissions for years after you grant them).
The Full Details option will allow others to open the appointments on your calendar and see the contents. This can be dangerous if you attach e-mails about performance reviews, personal medical information, or other sensitive information to a calendar invite. It is a best practice to give the minimal permissions needed for your co-workers to do their job.
The “Publish Online” option allows you to publish your calendar to Office.com, and has similar privacy settings. I would be very leery of publishing anything other than “Availability only” to the broader internet. I would also recommend using the “Only invited users can subscribe to this calendar”.
The final (and important) button on the calendar ribbon is “Calendar Permissions”. This is where you can check and set permissions that have been granted. I see that I have changed my default permissions to allow for people to be able to see my meeting subjects and locations, which is probably more information that I really want to share with 90,000 of my closest friends at Microsoft. I also see that I have 30-40 people with explicit permissions to see my calendar, many of whom are now on other teams or that do not a good reason to be able to see my calendar. This is where you will want to remove their explicit permissions or give lower permission levels.
Finally, there may be appointments (such as the aforementioned embarrassing doctor appointment) that you do not want to share with anyone, regardless of their permissions. You can open any appointment in Outlook, and on the right hand side of the ribbon, there is a small “Private” option. Click this, and only you will be able to see the appointment. Everyone else will just see “Private Appointment” on your calendar.