Air resistance is the friction that objects experience as they move through an atmosphere made up of air. Air is made up of relatively widely spaced particles in the form of molecules and atoms. These minute particles, en masse, must be pushed against for any object to move.
Since the particles are widely spaced and not strongly bonded to each other, they are easy to push against and thus allow for movement, but air resistance does become a serious force under certain conditions. High speed and a broad surface area increase the number of collisions made between an object and the molecules of air, thus increasing air resistance substantially. Air resistance acts in the opposite direction of an object's travel, slowing it down. This requires more force to achieve the same speed than if air resistance was not a factor. Aerodynamic designs for cars strive to reduce air resistance as this slowing effect results in reduced fuel efficiency.