Cyclocross is an off-road circuit race ridden on modified road bikes. The race usually consists of many laps around a short course full of obstacles that the rider must dismount and carry their bike over or through. The bikes are very similar to road bikes with the exception of knobby tread tires for traction and cantilever style brakes.
I've never competed in cyclocross myself, but here is a blog with a little more insight: Cyclocross
Also, check out this new article for more information about Cyclocross: Active: Cyclocross 101
I have just started learning about cyclocross myself and I have definitely found it interesting. It looks like a lot of fun.