An ordinary year has 365 days. A leap year has 366 days. The extra day is added to the month of February. In an ordinary year, February has 28 days. In a leap year, it has 29 days. This extra day is called a leap day.
In a leap year, February starts and ends on the same day.
A leap year can always be evenly divided by four. For example, 2012 was a leap year. But a year is not a leap year if it can be evenly divided by 100 but cannot also be evenly divided by 400. This is why 1600, 2000, and 2400 are leap years, but 1700, 1800, and 1900 were not.
Without leap years, the seasons would start one day earlier on the calendar every four years. After 360 years, spring (which usually begins on March 21) would begin on December 21 (which is when winter presently begins).
A number of countries use a lunar calendar (based on the moon). They have leap years when they add an extra lunar month. Different calendars add the extra month in different ways.