A leap year is a year in which an extra day is added to the Gregorian calendar, which is used by most of the world. A common year has 365 days, but a leap year has 366 days. The extra day, February 29, is added to the month of February. In a common year, February has 28 days, but in a leap year it has 29 days. The extra day, called a leap day, occurs on the same day of the week as the first day of the month, February 1.
In order for a year to be a leap year, it must be evenly divided by 4. Therefore, the years 2016, 2020, and 2024 are leap years, but the years 2017, 2018, 2019, 2021, 2022, and 2023 are common years. Most years divisible by 4 are leap years. However, any year that can be evenly divided by 100 is not a leap year unless it can be evenly divided by 400. This is why 1600, 2000, and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200, and 2300 are common years, even though they are all divisible by 4.
We have leap years because instead of 365 days, the Earth really takes a few minutes less than 365-1/4 days (365.24219) to go completely around the Sun. Without leap years, the seasons would start one day earlier on the calendar every four years. After 360 years, spring in the Northern Hemisphere and autumn in the Southern Hemisphere would begin on December 21 (which is when winter in the Northern Hemisphere and summer in the Southern Hemisphere presently begins).
A number of countries use a lunar calendar (based on the Moon, instead of the Sun, like our solar calendar is). They have leap years when they add an extra lunar month. Different calendars add the extra month in different ways. So a year which has 366 days instead of 365 days where the month of February has 29 days is called a leap year.
In a leap year, the corresponding months are January, April, and July, February and August, March and November, and September and December. No month corresponds to May, June, or October.
In the Gregorian Calendar, 97 out of every 400 years are leap years. In the Julian Calendar, 100 years out of every 400 are leap years. All other years are common years.