Cyclocross is a form of bike racing that's getting more popular every year. If you go to a 'cross race, you'll see people riding what looks like a road bike with knobby tires over grass, dirt, and gravel. During the race, riders must dismount and carry their bikes over barriers or up steep hills. Races are short and intense, and are great fun for riders and spectators.
But cyclocross bikes are not just for racing! If I could only own one bike, it would be a 'cross bike. They have the efficiency of a road bike, but the more stable geometry and wider knobby tires makes them extremely versatile. I can leave my house, zip along the road, then turn down a trail through the woods, which pops me out on another road. It's the most fun I've had on a bike. Plus, the tire clearance makes it easy to install fenders and create an all-weather commuting bike.
Brian Rutledge of Belmont Wheelworks says: