The following is from Wikipedia.org's article on Cyclocross

Cyclocross is a form of bicycle racing. Races take place typically in the autumn and winter (the international or "World Cup" season is September-January), and consists of many laps of a short (2.5–3.5 km or 1.5–2 mile) course featuring pavement, wooded trails, grass, steep hills and obstacles requiring the rider to quickly dismount, carry the bike whilst navigating the obstruction and remount in one graceful motion. Races for senior categories are generally between 30 minutes and an hour long, with the distance varying depending on the ground conditions. The sport is strongest in the traditional road cycling countries such as Belgium (and Flanders in particular) and France.

Cyclocross has some obvious parallels with cross-country cycling and criterium racing. Many of the best cyclocross riders cross train in other cycling disciplines. However, cyclocross has reached a size and popularity that racers are specialists and many never race anything but cyclocross races.

 

Cyclo-cross bicycles are similar to racing bicycles: lightweight, with narrow tires and drop handlebars. However, they also share characteristics with mountain bicycles in that they utilize knobby tread tires for traction, and cantilever style brakes for clearance needed due to muddy conditions. They have to be lightweight because competitors need to carry their bicycle to overcome barriers or slopes too steep to climb in the saddle. The sight of competitors struggling up a muddy slope with bicycles on their shoulders is the classic image of the sport, although unridable sections are generally a very small fraction of the race distance.

Compared with other forms of cycle racing, tactics are fairly straightforward, and the emphasis is on the rider's aerobic endurance and bike-handling skills.

A cyclocross rider is allowed to change bicycles and receive mechanical assistance during a race. While the rider is on the course gumming up one bicycle with mud, his or her pit crew can work quickly to clean, repair and oil the spares. Having a mechanic in the "pits" is more common for professional cyclocross racers. The average cyclocross racer might have a family member or friend holding their spare bike.

 

Checkout the CrossPrimer on Cyclocrossnews.com here
Another great article by Patrick O'Grady can be found here
MadCross.org has some great intro videos too...check out the Cyclocross National Chapmionship video from 2006 here
Check out this great movie showing a slippery world cup race in Europe!
Check out this video of proper barrier technique...here.