My knowledge is biased because I learned about it in the USA but my understanding is that it's a form of off-road running. It's not as gnarly as trail running, surfaces are slightly smoother and they have the odd obstacle (like a bale of hay) or mud to run through. I believe you tend to run it in teams as well as opposed to as an individual. There's a whole CC series, I think. I really fancy having a go at it.
Anyone know more about it? I feel a little 'vague' here.