As whole calendar days between the two dates you pick, ignoring time of day. If you pick the same date twice, the result is 0.
Total days is exact but not always intuitive over long spans. The years/months/days breakdown is calendar-aware — e.g. January 31 to March 1 is "1 month, 1 day," not a rough division of total days by 30.
Yes. Dates are normalized to midnight UTC before any math happens, so a day is always exactly 24 hours in the calculation — DST transitions can't add or subtract a stray day.