Cyclocross is a very specific type of bike racing. For the most part, the course is off-road but there are sometimes portions of pavement included in the course. You can expect to encounter grass, mud, gravel and a whole host of other assortments and combinations.
Races are based on a set time (measured by numbers of laps), not distance. Depending on your category, a race can be as quick as 30 minutes (for beginners), or as long as 60 minutes (for pros).
In most cases, race officials wait to see how quickly riders complete the first lap before deciding how many total laps will be completed. Depending on course conditions and pack motivation, the total time may be a few minutes less or more than planned.
Courses typically, but not always include barriers, as well as other ‘features’ that will necessitate hoping both off, and back onto, your bike. Officially, these barriers are considered obstacles that are no more than 40cm tall, and 4-6m apart. With very few exceptions, barriers are small enough for everyone to safely run through. Those brave (and skilled) enough can hop their bikes right over.
Often there are hills and muddy sections that are too difficult or slow to ride, and thus a racer will shoulder their bike and run with it. Running has become a smaller part of the racing over the last decade, but you may find mud so thick it’s often faster to run.