A year is determined by how long it takes the Earth to revolve around the sun. It usually takes 365 days. A leap year occurs every four years to help synchronize the modern day Gregorian calendar with our trip around the sun. It really takes us about 365 days and a quarter to revolve around the sun, so every four years, a day, February 29th, is added to the calendar to make it correct. If we didn't have a leap day, every 100 years the calendar would be wrong by about 24 days.
The next leap day is in 2016. People who are born on February 29th can claim that they aren't really as old as they say the are since their birthdays only occur every four years!