An organization similar to the Society for Creative Anachronism, except that they don't belive in letting the messy details of reality bog them down. They call themselves a Live Action Role Playing Game (LARP for short) but really they are more of a live action wargame.
The basics are as follows:
They have a class systems (Warrior, Wizard, Healer, Assassin, etc) that you progress through via weeks of attendance. Twelve weeks equals about a level, and there are six levels in each class. Meaning that people who have been around longer have more power on the field.
Battlegames are fought by teams picked in the usual high school gym fashion and then they go out and fight, usually with some objective (last man on the hill, capture the flag, kill the leader, etc)
There are also extravagant feasts and courts where people dress up in quasi medival garb and pretend they are more important than they are in real life.
Amtgard is truly a place for people who are too geeky for the SCA.