The Julian calendar was introduced by Julius Caesar in 46 BC. It was supplanted by the Gregorian calendar beginning in 1582, although some countries continued using the Julian calendar until the 20th century.
The Julian calendar has 365 days per year, with an extra "leap day" every 4 years. Unfortunately, adding a leap day every 4 years makes the average Julian calendar year 365.25 days long, a slight error from the more accurate 365.2424 days. While the difference seems small, the Julian calendar falls behind the astronomical year 1 day every 128 years.
For general information about date and calendar issues of interest to genealogists, see Dates And Calendars. For an article discussing the effects of the Julian and Gregorian calendars on genealogical records, see Calendar Change - 16th Century. For a detailed description of the Julian calendar, see WikiPedia.