DAVdroid is a CalDAV/CardDAV client. If you just want to subscribe (= download and import at regular intervals) an iCalendar (.ics) file / Webcal feed, we suggest to use ICSdroid.
Since DAVdroid 1.8, DAVdroid can detect CalDAV-advertised Webcal feeds and integrate them using ICSdroid.
WebCal (.ics subscription) events are stored in an .ics (iCalendar) file (often generated dynamically) which can be fetched and imported using HTTP. Only one direction, no special protocol except HTTP.
CalDAV is a sophisticated WebDAV- (and thus HTTP-)based protocol for synchronizing events and tasks in two directions (RFC 4791 and others)