A cyclocross bike is built a bit differently from a standard road bike - with tweaks made to create a machine that's more suited to off-road riding.
These include a slacker head angle, for better off-road handling, and a higher bottom bracket to avoid too many accidents involving rocks and roots.
The wheelbase will often be longer, creating greater stability, and the top tube (often flattened for ease of shouldering) is often a little shorter to provide a more upright riding position. The bars and saddle are usually closer to being level, allowing the rider to shift their weight around more easily to tackle obstacles and to control traction.
Of course, all of the above sits on a sliding scale – more aggressive race orientated cyclocross bikes will sacrifice some security for a more nimble ride quality, whilst the closer they get to the adventure road category, the greater the stability on offer.