A leap year occurs approximately every 4 years and has 366 days versus the common year has 365 days. The extra leap year day is added to the to the month of February as the 29th day. Leap years in our current time past/future are the years 2008, 2012, 2016, and 2020.
Details on Leap Years
Under the Gregorian calendar most years that are multiples of 4 are leap years with some exceptions like years that are multiples of 100. It takes the earth slightly less than 365.25 days to go around the sun which is called the tropical year. That is almost 6 hours (5 hours, 48 minutes, 46 seconds) longer than the 365 day Gregorian calendar. So to offset the difference the calendar has adopted leap years to occur almost every 4 years (February 29) to make the rotation of the earth around the sun match the calendar days, months and years.
Leap years and the Julian calendar which eventually led to the Gregorian calendar, were introduced by Julius Caesar. The leap year used was years that were multiples of 4 but over time did not keep up with the earths rotation around the sun. Eventually the Gregorian calendar was adopted from the Julian calendar in 1582 and corrected the leap year problem.