The year is 365.242374 days or 365 days, 6 Hours, 49 minutes, 12 seconds.
The gregorian calendar, (the one we use) is regulated to keep spring equinox at about March 21. To correct for the fraction of a day that adds up with each passing year is corrected with the leap day.
Because the year is not exactly 365¼ days we have to make more corrections than just a simple day every four years.
Leap years are years that are multiples of 4, except
If the year is a multiple of 100 it is not a leap year, except
If the year is a multiple of 400 it is a leap year.
This is not to be confused with the leap second which corrects the speed of the Earths rotation on its axis.
The orbit of Earth is slowing, so we may need to make an adjustment in a couple thousand years.