2004 and 2008 will both be leap years. For hundreds of years, people used a calendar called the Julian calendar that followed this rule, adding a leap year every four years. However, because .9688 isn't exactly a whole day, the Julian calendar slowly began to disagree with the real seasons. In 1582, Pope Gregory fixed this problem by ordering everyone to use a new set of rules. These rules are named the Gregorian calendar, after him.
Every fourth year is a leap year.
However, every hundredth year is not a leap year. 1900 and 2100 are not leap years. Every four hundred years, there a leap year after all. 2000 and 2400 are leap years. People in English-speaking countries didn't start using the Gregorian calendar until 1752.