Here's a brief answer to your Q, Jeff --
Leap years are years with 366 days, instead of 365 days. The actual length of a year is 365.242 days. Leap years occur every four years, and years that are evenly divisible by 4 (2004, for example) have 366 days. This extra day is added to the calendar on February 29th.
For a detailed explanation, please browse Wikipedia.