The SCA is the Society for Creative Anachronisms. In case you don't know an Anachronism is something out of time. For example if you are watching a period movie such as Brave Heart and one of the characters has a wrist watch on, the wrist watch is an Anachronism within the movie. Within the Society the Anachronism is the players. I know this doesn't really answer the question, but it sets the tone for how the name came about.
The SCA is oft quoted as "The Middle Ages as they should have been". We are reinactors who don't necissarily bother with the trivial details of actual events but rather the time itself. That is not to say we aren't interested in actual historical facts, but rather that we are interested in the way things worked and not the geography or actual battles and such.
The SCA started in the late 60's by a bunch of sci-fi/fantasy geeks (writers and readers) at a birthday party in which they hosted a middle ages themed tournement. From there it progressed to over 30,0000 members world wide. For more information visit thier website.
The SCA is a global organization, and it has been devided up into The Nineteen Kingdomes: