Leap years happen when we add one day to the end of February in order to align our calendar with the Earth's orbit. Once every four years, we tack a 29th day onto the end of February, which is usually 28 days long, making a leap year 366 days instead of 365. We generally refer to 365 as the number of days it takes for the Earth to orbit the sun, but it actually takes 365.242190 days or 365 days, 5 hours, 48 minutes, and 56 seconds, to be exact. To compensate for using a rounded number the rest of the time, we add this extra day periodically to keep our calendar in line with our equinoxes, solstices, and seasons.

Source: USA Today