The modern cylocross bike concept as it is known today, evolved from the country and "rough-stuff" bikes of earlier years. The very best classic bikes that were used for 'cross type riding and steeple-chase events were the touring bikes with large tire clearances and wide brake set-ups, or cantilever style brakes. The relaxed geometry of the touring bikes' frame made riding off road easier, and the wider clearances made it easier to clear debris, dirt, and mud.
Here is an article from someone who knows much more than I do about the bikes used (and everything else 'cross related, from the early years until now): http://bicyclespecialties.blogspot.com/2008/11/cyclo-cross.html
Overview
Cyclocross, or CX, combines the speed and performance of road biking with the mud-gripping rolling efficiency of mountain biking. CX was developed to keep road racers fit during the fall and winter, but evolved into its own type of racing. Cyclocross racing features short courses with various types of terrain, including dirt, grass and mud; and obstacles that force riders to dismount and carry their bikes. Because these bikes are designed for performance, both on and off road, CX has several advantages.
Frame
Cyclocross frames are designed like road frames to be light and easy to carry, but slightly different. They are more durable and feature wider spacing in the stays to prevent mud from jamming up the wheels. The bottom bracket is higher off the ground for more clearance over obstacles and better maneuverability around turns. CX frames also keep the rider at a different position than road frames -- slightly more upright and closer to the handlebars
Wheels
The rims on a CX bike are road standard 700c with wider tires for better grip off-road. The tires have treads that provide traction in dirt and mud, but still smooth enough to roll efficiently on the road with little wheel resistance.
Components
Cyclocross bikes have gears similar to road bikes, but because CX racing does not reach as high a speed, the gearing is typically easier than road biking. Gearing on a cyclocross bike usually will have a single or double chain ring in the front and wide range of gears in the rear. Cyclocross bikes feature drop handlebars like those of a road bike. Cantilever brakes are used for better stopping power and more clearance for the wider tires.
Capabilities & Uses
Because cyclocross bikes handle well on and off road, they are superior for adrenaline junkies who are looking for a new challenge. CX racing is shorter and slower than road racing, so it is fine for beginners. And because cyclocross bikes can handle several types of terrain efficiently, they will work well for commuting, city riding and road racing. Except for the most extreme types of terrain, they handle well off-road as a do-anything bike for a do-it-all rider.
