Okay then, a leap year (or as it's otherwise known, an intercalary or - don't laugh - bissextile year) is a year which includes an extra day. Why the extra day? To keep the calendar year in line with the astronomical year, that's why.
The problem is that it actually takes the earth six hours less than 365 days to orbit the Sun. We compensate for this by adding - or to use the correct scientific term 'intercalcating' - an extra day to any year that's divisible by four.
And if you're not one for maths, you might prefer to remember which years are leap years by the fact that the Summer Olympics always take place when there are 366 days in the year.
One last thing - as complicated as the above might sound, it only applies to the common solar - or Gregorian - calendar. The Chinese, Hindu and Hebrew calendars are intersolar which means that rather than adding an extra day every four years, they add an extra month.
To say any more would require talking about mathematics so complicated it would leave Brian Cox rocking backwards and forwards in the corner of a padded cell. As complicated as this whole leap year business might be, just take it from me that it could be a whole lot worse.