Mud, and more mud.... traffic free circuit racing with a soft-ish crash landing etc.
Bikes: traditional cyclo X bikes usually have drop bars, cantilever brakes and often utilise bar end shifters for changing gear. A lot of people ride informal 'fun' events on hard tail mtbs though, or in fact any bike that will get round a lumpy muddy field without beating you up too much.
Went to watch a local club event once where there appeared to be a fairly even mix of men, women and children doing the course. It looked like it was great fun and a good workout on a winters morning.