What's an Iron Butt?

I spent the last couple of weeks taking some SAP classes in Philly. I met Stephan Bolduc, a French Canadian from Laval, which is just North of Montreal. He said he had completed the Iron Butt Rally just this summer. Of course with his accent, I didn't really understand what he said, so I asked him to explain. It turns out that extreme comes in all shapes and sizes, not just for the young, but for the young at heart.