As much as the Nextel Cup is an individual sport, successful racing at the larger tracks in particular can't be accomplished alone.
Here's an explanation about why the cars travel so close together and how cars can team up to punch through the air to help one another through drafting.
Aerodynamics: Study of airflow in regard to a stock car.
Drag: The resistance a car experiences when passing through air at high speeds.
Downforce: Downforce can be altered to improve the car's grip or traction by adjusting the spoiler as well as other aerodynamic changes to the car and its setup. As downforce is increased, the grip/traction is increased as well as tire wear. Increasing downforce comes at the expense of creating more drag, which will reduce fuel efficiency.
Draft: The aerodynamic effect that allows two or more cars traveling nose to tail to run faster than a single car. When one car follows another closely, the one in front punches through the air and provides a cleaner, less resistant path for the trailing cars.
Drafting: The practice of two or more cars running nose to tail to create more speed for the group. The lead car displaces the air in front of it, creates a vacuum effect between its rear end and the nose of the second car and pulls the trailing cars along with it with less overall resistance. Two or more cars drafting will travel faster than a single car.