Though the sport has existed for decades, I had no exposure to cyclocross until I moved to Portland. And even after I first heard about it, I had only a vague picture in my head of what cyclocross entailed: something like a modified steeplechase with bikers leaping over hurdles and falling into the mud. It wasn't until I went to my first race a few weekends ago that I really “got it”.
Cyclocross courses vary in time (anywhere between 30 minutes and an hour), location (wherever), terrain (pavement, trail, grass, mud, hills, gravel), and obstructions (various barricades and boards to leap over). Participants compete on a short course (1-3 miles) that they ride over and over for a set amount of time; the repetition allows competitors to assess the course and strategize on successive laps. Because of the staggered start, it's impossible for racers to know where they stack up against other competitors; their only goal is to complete as many laps in the set amount of time as possible, while avoiding careening bikers, trying not to pull a muscle when hurdling over an obstacle, and maintaing focus.
Cyclocross bikes are similar to road bikes (or are converted road bikes), with a focus on a light-yet-strong frame and thin tires with good traction.
These photos are from the Cross Crusade Series Race 3 at the Portland International Raceway. You can view a full schedule here; this coming weekend's race is back at the Portland International Raceway, but with a different course.