Calendar is much more than just a calendar. It allows you to keep track of appointments, both one-offs and ones that repeat, you can set alarms or make additional notes on any entry.

Links to Contacts

If you show the birthdays calendar in Calendar, it collects its data from the entry in Contacts - you cannot alter birthdays directly in Calendar.

Synchronising with other devices

It's easy to synchronise your calendars with your iPhone, iPad or iPod Touch using either wirelessly using iCloud or with a cable and iTunes. This way, any additions or changes you make on one device are also made on the other(s).