The calendar provides the ability to add, appointments, events and courses.
Appointments
These are for say a wedding appointment in the shop. This is primarily for your own reference
Events
This is more related to if you are attending or running an event, for example a wedding fair
Courses
These are if you are running a course where you would like to register or charge attendees, for example running a course on how to create wedding bouquets
For more information see
Creating events in the calendar