Currently, the GNOME calendar uses the vCal file format standard for storing and loading its appointment and todo database. This standard is widely used in various other calendar applications, so it provides a good mechanism for exchanging appointments.
In the future the GNOME calendar will support the upcoming iCalendar specification as well.
If you want to know more about the vCal specification, you can check the Internet Mail Consortium (http://www.imc.org) for the details.