[Angrybird 12]

From E-How. com

Purpose of a spoiler is to Reduces Overall Wind Drag
A spoiler is a structure built into the body of car, usually on the back of the car behind the rear window, which is designed to reduce wind resistance by disrupting inefficient airflow. The faster a car drives, the more it must cut through the wind, and the more air resistance the car experiences. Generally the more surface area that is exposed to the wind, the more drag the car will generate, meaning the added surface area of the spoiler adds some drag. On the other hand, a spoiler diffuses airflow in such a way that drag is reduced on other areas of the car, resulting in an overall reduction in drag and increase in aerodynamic efficiency.

Wing-Shaped Spoiler
The term spoiler is commonly used interchangeably to describe an aerodynamic enhancement, which is a true spoiler, and a wing shaped extension, often seen on sports cars or race cars. The wing shaped extension functions differently from a standard spoiler. It uses airflow to create downward force on the body of the car to increase friction and traction, thereby allowing the car to drive faster and reducing the tendency for a very fast vehicle, such as a drag racer to lift up from the ground. Unlike a normal spoiler, a wing type spoiler creates a net negative of in terms of wind drag, so the efficiency gained in terms of traction must exceed the extra wind drag in order for the spoiler wing to yield a net benefit.

Decorative Spoiler
While the standard spoiler and wing spoiler are aimed at increasing driving efficiency, many car spoilers are installed as a matter of aesthetics. Many modern vehicles come with spoilers aimed at making the car look stylish or sporty while yielding little or quite likely negative wind drag efficiency. Automotive enthusiasts sometimes install their own spoiler modifications. Such modification is much more likely to result in lower aerodynamic efficiency, speed and acceleration than it is to result in an increase in performance.

And from Bodykits.com

Diffuser - A diffuser is a carbon, plastic, urethane or fiberglass peice that mounts on the rear bumper over or near the exhaust tips. A diffuser helps air travelling underneath the vehicle join air flow rolling over the top of the vehicle in a more aerodynamic manner.

Spoiler - A spoiler is an aerodynamic device attached to an automobile whose intended design function is to 'spoil' unfavorable air movement across a body of a car in motion. This can result in improved vehicle stability by decreasing lift or decreasing drag that may cause unpredictable handling in a car at speed. Spoilers are often fitted to race and high-performance sports cars, although they have become common on passenger vehicles, as well.