When I was a kid, I would ride my bike around the neighborhood with my brother and our friends. A lot of times, we would race over courses that included "obstacles" like bunny hops and little muddy areas. Sometimes, the course would go through a sand box or grassy patch and it would just make more sense to pick your bike up and run. Other times, we'd have somebody's little brother or sister stand at a set point on our "course" and throw things at us to add to the challenge.
If we had stuck with it, we could've maybe become really good at Cyclocross. Marc Bavineau stuck with it and good for him because he recently won the USA Cyclocross championship in Kansas City.
Here's a video that explains what's going on. Strangely, it's described as "an hour in hell". I think I might've described it as, "what me and my friends would do on Saturday afternoons in 1987". Enjoy!
Update [2008-1-4 13:3:30 by Dex]: More Cyclocross coverage from Podium Cafe.