Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object as defined by wikipedia.
Aerodynamics of automobiles:
drag:
drag is a fluid dynamics term, sometimes called as air resistance or fluid resistance. It is defined as "forces that oppose the relative motion of an object through a fluid. Drag forces act in a direction opposite to the oncoming flow velocity". (wikipedia)
| Shape and flow | Form drag | Skin friction |
|---|---|---|
 | 0% | 100% |
 | ~10% | ~90% |
 | ~90% | ~10% |
 | 100% | 0% |
lift:
A fluid flowing past the surface of a body exerts a surface force on it. Lift is defined to be the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is defined to be the component of the surface force parallel to the flow direction.(wikipedia)
Note: In aircrafts Lift/drag ratio is expected to be higher. But this is not the case in automobiles where lift is to be reduced and drag too to be reduced. What is especially required in an autombile such as a racing car is the downforce (as opposed to the lift required in aircrafts).
Upper: non-aerodynamic car; Lower: aerodynamically improved design
From: http://www.generalamherst.com
A diffuser is seen in most racing cars:
Rear diffuser of Porsche - Courtesy: Wikipedia
Rear spoiler of Toyota - Courtesy: Wikipedia
Front Air Dam of Mazda - Courtesy: Mazda
Please visit hotrod for a fine detailed description of the role of aerodynamics in automobiles!
