Its a 50 year old re-enactment world wide re-enactment society, that roughly re-creates the time period between 1000 and 1600 'as it should have been'
Which is kind of hard to explain. Its very open in that some people play it like a ren-faire, and other people take it very seriously and many things inbetween.
I did it for about 10 years, its where I met my wife. It WAS my life for many years.
The most distinctive thing it does is full contact, full force armored combat, using practice weapons (but with the same weight as the real thing) against real, full weight armor, in battles with up to a thousand people. (but usually dozens to hundreds)
It a combination cult, martial art, craft fair and drunken party. The biggest difference between it and a refaire is that not a performance, its not for other people, other people usually aren't even allowed in, its to LIVE in a certain way, by certain standards that are different from the modern world.
In the time I did it, I broke three bones....all of them on other people.