The Gregorian calendar is the most widely used civil calendar in the world today.  It is a reform of the Julian calendar.  The calendar revision was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582.  This well after the fall of Rome in 750 and the East-West Schism of 1054.  

Years in the reformed calendar continue the numbering system of the Julian calendar, which are numbered from the traditional Incarnation year of Jesus.  Traditionally these have been called "anno Domini" (AD) but in recent times CE for "common era" has become common although some use "Christian Era".  (This system had been in place since 525.)

The changes made by Gregory corrected the drift in the civil calendar which arose because the mean Julian calendar year was slightly too long, causing the vernal equinox, and consequently the date on which Easter was being celebrated, to slowly drift forward in relation to the civil calendar and the seasons.

The Gregorian calendar system dropped 10 days in its first year to bring the calendar back into synchronization with the seasons and, to keep it there, by adopting the following leap year rule:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 was not a leap year; the year 2000 was a leap year. 

In the Julian calendar, all years exactly divisible by 4 are leap years.