Keeps track of events and appointments, allows multiple calendar views (such as calendars for "home", "work", and "kids") to quickly identify conflicts and free time.
Is integrated with .Mac, so calendars can be shared over the Internet. You can also share calendars via the WebDAV protocol.
You can 'subscribe' to other calendars so you can keep up with friends and colleagues, and other things such as athletic schedules and television programs.
Allows reception of notification of upcoming events either on screen, by mail, SMS, or pager.
See also: ICalendar RFC 2445