iCalendar versus CalDAV

an .ics file (iCalendar) is for importing static data into a calendar.

CalDAV is a protocol extension for WebDAV
for sending and receiving updates and can used to manipulate data in the iCalendar format.

Many people mix (unjustified) tthe the usage of both

CalDAV is like the HTTP for calendar stuff, and iCalendar is like HTML.