=DAYS(end_date, start_date) returns the days between two dates. Note the order: end first. It's the same as subtracting, =C2-B2, but it reads clearly.
=DATEDIF(start_date, end_date, unit) counts whole units between two dates:
"D" days, "M" whole months, "Y" whole years;"YM" the months left over after the whole years, "MD" the days left over after the whole months.
DATEDIF is an old function that Excel keeps for compatibility: it has no AutoComplete entry in some versions, so type it in full. The start date must not be after the end date, or you get #NUM!.
Why not divide by 365? Leap years and birthdays: (C2-B2)/365 drifts and can round up a year too early. DATEDIF counts calendar years.