iCal is a way to have a web based calendar and schedule sharing. It is implemented/supported by a large number of products, including the Apple iCal application, Chandler, Lotus Notes, Korganizer, Mozilla Calendar (including Mozilla Sunbird), Mulberry, Ximian Evolution, and, to an extent, Microsoft Outlook. To subscribe to the Duke Colloquium iCal, click on the link and open the file with your favorite calendar/scheduling application, or copy and paste the link into it.
Note: If you are using one of the Physics Department's computers, a calendar program that supports iCal is already installed! All you should need to do is click the link and let your browser launch the appropriate external application if prompted.
More information can be found http://www.answers.com/ical and here.