A calendar is a collection of events along with a list of people who may see, edit or administrate that list and a default color.
You start with an empty calendar (your primary calendar) and can add events and share it with other users.
You can add more calendars, either calendars shared with you by other users, external calendars from url or freshly created calendars.
Calendars you have write access to will show up as "My calendars" (no matter who actually created it) and read-only calendars will show up as "Other calendars"