General Overview of Superchargers
Superchargers are an external mechanism driven off the engine's auxiliary drive belt. The mechanism can work in many fashions, but all have the same basic effect: to increase the force on the incoming air to the engine. Since superchargers are belt-driven, they do create small amounts of parasitic drag on the engine, however the effects of the supercharger greatly outweigh the drag.
Generally, superchargers work with gear ratios to create the desired speed of the impeller (or other air-moving mechanism). If less boost is desired, a larger drive-pulley can be interchanged onto the supercharger. If greater boost is desired, a smaller pulley is used. However, boost levels can be controlled in other ways too. A wastegate or blow-off-valve can be used in conjunction with a correctly sized pulley to have great control over boost levels.
A centrifugal supercharger is the most basic, and most common type of supercharger seen on street cars today. A centrifugal supercharger works a lot like a fan. The supercharger consists of an inlet port, an impeller, a scroll, and a discharge port. The air comes in the inlet port, and is hit by an impeller. The impeller must spin at speeds of 40,000 - 60,000 rotations per minute in order to create boost. At idle speeds, the impeller does not have enough rotational speed to produce any boost. The impeller utilizes centrifugal forces in order to produce boost.
The impeller is the integral part of the centrifugal supercharger (depicted as black fins). As the air comes in at the center of the compressor blades, the impeller grabs the incoming air from the inlet port (1). Since the impeller is turning at tens-of-thousands of revolutions per minute, the air is naturally thrown back and towards the outskirts of the fins due to centrifugal forces created by its rotational inertia (2 & 3). "At the outside of the blades, a "scroll" is waiting to catch the air molecules. Just before entering the scroll, the air molecules are forced to travel through a venturi (depicted as the larger grey circle), which creates the internal compression. As the air travels around the scroll (4), the diameter of the scroll increases, which slows the velocity of the air, but further increases its pressure (5)."2
While a centrifugal supercharger is capable of very high levels of boost and high levels of horsepower increase, the boost doesn't occur until high RPMs are reached (normally the supercharger starts creating boost around 3000 RPMs).
1 - http://www.competitionplus.com/02_12_2003/superchargers.html
2 - http://www.superchargersonline.com/content.asp?ID=76