A regular mountain bike will do if you're a cyclocross beginner.
Cyclists considering competing in a cyclocross event don't need a special bike. Most mountain bikes, unless they have forward-facing handlebars, suffice—at least for beginners.
More experienced cyclocross athletes, like many road, mountain and track cyclists, seek and benefit from more sophisticated machines.
Since bona fide cyclocross riders appreciate inclement weather and muddy courses, a road bike frame combined with wider tires and equipped with cantilever (padded) brakes, often called "V" brakes, is ideal. The wider tires and cantilever brakes allow more traction and optimal braking and mud clearance.
There is an evolution in gear going on, too.
"Actually, we're seeing more cyclocross bikes with disc brakes," says Bob Rolke, manager of The Bicycle Business in Sacramento, California.
"It's come from the mountain bike world and it's particularly gaining popularity in parts of the country where the weather is really bad. Of course, disc brakes can get wet, but it's easier to keep them mud-free." (The Union Cycliste Internationale, cycling's governing body, bans disc brakes in its events.)
Cyclocross riders are required to carry their bikes over various course obstacles, so there's a fine balance between a sufficiently sturdy bike that's also lightweight.
Other important considerations include: dual-sided pedals to help efficient mounting and dismounting, and low gearing for the short, steep climbs common on cyclocross courses.