A clutch kit comes with it a clutch disc or a clutch friction disc and clutch pressure plate, although some clutch kits have pilot bushings or pilot bearings. The pressure plate contains a clutch plate, springs, cover and release fingers. Of the two types of pressure plates, the diaphragm-type is used more often than the spring-type pressure plate.
The clutch disc, on the other hand, is a metal disc that directs the flow of power between the engine and the transmission along with the pressure plate and the flywheel. Most clutch kits have one or more friction discs. In cases where there is more than one, the discs are pressed tightly together against the flywheel by springs. Regular clutch discs are lined with asbestos, the same material that is used in brake shoes and pads to achieve friction. This special facing enhances the grip between the flywheel and the disc. Clutches used by trucks and racing cars, however, use ceramic for their clutch discs for increased friction.
If a clutch has better materials used for friction and stronger springs to go with it, chances are you get faster and harder launches, quicker shifts, and much better durability.