This FAQ is intended to explain many things about the SCA in a short and concise manner. You can find more verbose FAQs on the Official SCA FAQ and everywhere on the internet. Another helpful article is Wikipedia.
That's the Roman version of today's date with the SCA version of the year. To clarify on that point, rather than 'A.D.' or 'C.E.' we go by 'A.S.' or Anno Societatis. Latin for 'in the year of the society.' More information can be found here. More information on how Rome did their dates can be found on Google.An interesting feature of this website is that you can input a mundane date to have the website display the corresponding SCA date. So if you wanted to get the SCA date of 25-09-2018 then you'd just have to put in the url:
Kingdom: area ruled by a King and Queen (typically covering several U.S. states or Canadian provinces, and can be as large as countries or collections of countries). Minimum members required 400.
Principality: area within a kingdom ruled by Prince and Princess (large area sometimes comprising entire states). Minimum members required 100.
Region: equivalent of principality without ceremonial representative
Barony: area administered by a Baron and/or Baroness, the ceremonial representative(s) of the Crown (small chapter typically occurring in an urban area). Minimum members required 25.
Canton: local branch reporting through a barony (local chapter, which may be on the way to becoming a shire)
Province: equivalent of barony without ceremonial representative
Riding: local branch reporting through a province
Shire: local branch reporting directly to a kingdom or principality (local chapter typically occurring in rural areas). Minimum members required 5.
College: institutional branch based at a school, research facility, etc. (may be a part of a larger local group or report directly to a principality or kingdom)
Stronghold: institutional branch based at a military installation (may be a part of a larger local group or report directly to a principality or kingdom)
Port: institutional branch based at a military installation in situations where groups of members will be detached for long periods, as with ships at sea (may be a part of a larger local group or report directly to a principality or kingdom)
Other than the actual representative groups described above, there's two common versions of unofficial groups:
Guilds: Usually a group dedicated to some kind of artisan pursuit. Like a blacksmithing guild or some such.
Household: The catch-all term for 'other' groups. Could be a group of friends or a group of students and masters of a fighting art.