The
clutch system in most modern automobiles is comprised of two main parts, a
clutch release system and the actual
clutch assembly itself.
The release system is typically comprised of the well known clutch pedal everyone loves to ride with their left foot, a hydraulic assist system comprised of master and slave cylinders and clutch fluid lines, a release fork and a throwout bearing. When the user depresses the clutch pedal, pressure is sent through the clutch line from the master to the slave cylinder, which contains a piston that pushes on the release fork. The fork sticks down into the bell housing of the transmission and connects at its other end to the throwout bearing, which rides on the input shaft of the transmission and pushes against the splines on the clutch assembly.
The clutch assembly is typically comprised of two pieces. The first piece is a clutch disc that has friction material either bonded or riveted to each side. This friction material is located around the outer rim of the disc, and interacts with both the pressure plate and the flywheel. The pressure plate consists of a flat, smooth pressure surface connected to a housing that contains a set of splines. The clutch disc is typically mounted on the input shaft of the transmission and spins with it. The flywheel is bolted to the engine, and the pressure plate housing is bolted to the flywheel with the disc sandwiched in between. When the splines on the pressure plate are pressed on by the throwout bearing (due to you pressing on the clutch pedal), the pressure surface is backed away a very small amount which causes the clutch disc to be separated from both the pressure plate and flywheel. The disc can then spin freely on the input shaft of the transmission and no power is transferred. The flywheel (and the clutch pressure plate) are then the only pieces of hardware attached to and being spun by the engine.
There are many different types of clutch disc, but the most common types are either sprung or unsprung hubs. Sprung hubs contain a set of springs around the inside of the clutch disc which allow for the clutch to have that well known "range of engagement", allowing for smooth release and activation. Unsprung clutch discs are typically used more for drag racing types of applications as they have a more "on-off" type of behavior. Definitely not something you want to drive on the street.