Although these cycles may look like a road bike they are very different bikes. A Cyclo-Cross race rarely touches any road; the courses can be through trail sections as you would find on a mountain bike race. Obstacles and steps are often features of the course forcing the rider to carry the bike at times. Tyres are the same diameter as a road bike but have knobbles for grip like a mountain bike.
The carrying of the bike means that the top tube has become flattened so it is easier and less painful when on your shoulder. Compared to a road bike, a wider frame is required to allow for the knobbles of the tyres. Also the geometry alters. A longer wheelbase on many brands offers a more controllable, planted ride over rough terrain. The seat tube is also more relaxed to help the rider spread their weight over the bike through technical sections of the course.
An alloy frame is most common for Cyclo-Cross however carbon is used at the higher end along with some more niche steel and titanium models for those that want compliance and longevity out of their frame.
Componentry is used from both road and mountain bikes. The STI levers you see on road bikes are found on Cyclo-Cross bikes along with the front and rear derailleurs. Cassette sizes are similar to those on road bikes however the chainset most commonly carries a 46-36 chain ring setup with some riders opting for a single chain ring. Cantilever brakes are used along now with disc brakes as they are now UCI race legal many of the top brands released Cyclo-Cross frames with disc brakes straight away.
A great sport to get involved with if you ride often. The season runs through Autumn and Winter so many come off the road or mountain bike from the summer and jump straight on their Cross bike. Even if you don't wish to compete these bikes can be used as a great all-rounder. You can still get some good speed up on the road but you can mix it up a bit and jump on the trails when you like.