A road bike has a very light, streamline frame that is made for high speeds on the open road. The tires are very thin and the handles are made specifically so you can hunch your body over into a aerodynamic position. The price tag on these type of bikes can vary from a few hundred dollars up to thousands of dollars.
Well known athletes like Lance Armstrong use this type of bike in events like the Tour De France. It is a great tool for any athlete to do cardio workouts. Biking does not involve pounding on the knees like running, so it can be a great alternative.