The Gregorian Calendar is the calendar currently in use in most of the world. It was decreed by Pope Gregory XIII in 1582 and is named for him. Like the Julian Calendar, the Gregorian calendar has 365 days per year. An extra "leap day" is added every 4 years, except when the year is divisible by 100 but not by 400. As a result, 3 leap days are skipped every 400 years. The average Gregorian calendar year is 365.2425 days long, a close approximation of the actual length of the astronomical year.
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 discussion of the Gregorian calendar, see WikiPedia.