Julius Caesar introduced the first leap year around 46 B.C., but his Julian calendar had only one rule: Any year evenly divisible by four would be a leap year. That created too many leap years, but the math wasn't tweaked until Pope Gregory XIII introduced his Gregorian calendar more than 1,500 years later.
Caesar's concept wasn't bad, but his math was a little off; the extra day every four years was too much of a correction. As a result, there's a leap year every year that is divisible by four, but to qualify, century years (those that end in 00) must also be divisible by 400. So, the year 2000 was a leap year, but the years 1700, 1800 and 1900 were not.
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