The problem with most calendar systems is that they are based on 24-hour days, but the orbital period of the Earth is not an exact multiple of days. Over time, calenders get out of step with the actual orbital period. This is most readily seen in a changing date for the start of the seasons (or the occurrence of the solstices and equinoxes).
The Julian calendar was introduced to solve the problem of calendar drift. In a standard year, there are 365 days, but every four years there is a leap year of 366 days. The average year is then 365.25 days long, which is close to the (current) tropical year of 365.24219 days.
First used in the Roman Empire under Julius Caeser in 46 BC, the Julian calendar was a vast improvement on previous calendars. However, it still got out of step with the tropical year, resulting in an error of about 8 days after 1000 years.
The Julian calender has now been superceded by the Gregorian calendar, which was introduced by Pope Gregory XIII in 1582. The Gregorian calender restricted the number of years that were actually leap years: only century years (e.g. 1600, 1700, 1800) that were divisible by 400 were considered leap years. The Julian calendar was not discarded immediately: Britain waited until 1752, while Russia, Greece and Turkey continued with the Julian system until early in the 20th century.
The development of the Julian calendar is attributed to the Roman astronomer Sisogenes. A consequence of its introduction was to change the start of the year to 1 January, leaving the Latin names for September (7th month), October (8th month), November (9th month) and December (10th month) two months behind schedule!