A leap year occurs every four years, when February has an extra day (today). This day is called a leap day or intercalary day. Dr. Mary Singleton, St. Columba's College physics teacher explains the reason for this extra day.
A year in astronomical terms is the time taken for the earth to complete one full orbit of the sun. This time is 365 days, 5 hours, 48 minutes and 46 sec. This means that by having a year of 365 days, we are getting out of step by 5 hours 48minutes and 46 seconds each year. While this might not seem like much the result would be that over the course of a century the difference between the solar year and the calendar year would amount to 25 days, so relatively quickly the accepted seasons would get out of kilter. In order to avoid this situation an extra day is added to the calendar every four years, thus accounting for the overshoot.
However this is not completely correct. In fact by doing this the calendars are still out by 11 minutes 14 seconds. This would mean that every 128 years we would have gained an extra day. To correct for this an extra rule was introduced, that a century year is not a leap year unless it is evenly divisible by 400!