it's a short-circuit multiple lap race lasting one hour-plus-one-lap [or less] over a variety of terrain including, but not limited to, grass, dirt and singletrack. courses are designed to for riders to dismount and run [over obstacles, etc.] at least twice per lap [and up to four times]. courses are usually 2.5-4 km in length. cyclo-cross is raced in the fall and winter, when conditions are typically very bad. it evovled out of a winter training regimen for road racers that emerged inthe low countries and northern france after the second world war. the best way to describe it is a steeplechase for bikes.
cyclo-cross differs from xc mountain biking in a number of respects:
1. a big part of the sport involves dismounting and running with the bike. this, by the way, is the meaning of "cross" in cyclo-cross.
2. cyclo-cross is an anaerobic sport, xc mountain biking is an aerobic endurance sport. the best way to describe it is a one-hour sprint.
3. cyclo-cross courses are very short and twisty over extremely varied terrain, xc mountain biking courses are much, much longer.
4. despite the variety of terrain and surface, cyclo-cross courses are typically much less technical than mountain bike courses [though, in quebec, where most of the courses are designed by mountain bikers, the 'cross courses tend to feature rather more twisty singletrack than is normal].