The clutch isn't the most complicated mechanism on your car, but it does offer ample opportunity for failure. Most types of failure will come courtesy of normal wear and tear on the vehicle, but operator error has spelled doom for more than one.
Clutch friction surfaces are just like brake pads in all but shape, and the flywheel is essentially just a one-sided rotor. Clutch material will wear out just like the linings on your brake pads, and where it does, there's slippage. There's no repairing a worn-out clutch disc -- your only option is to replace the disc and possibly have the flywheel machined smooth if the clutch backing material scored grooves in it.
This happens more often than you might think; especially given the clutch's proximity to both the engine's rear main seal and the transmission's front seal. Oil leaking from the engine or transmission will eventually work its way into the friction disc, which soaks it up like a sponge. Following oil impregnation, the disc is worthless and headed for the landfill. Water impregnation can also cause clutch slippage but tends to be temporary unless you've slipped the clutch enough to burn it.
This phenomenon is somewhat related to brake fade and happens for the same reasons. Brake fade happens when a driver overheats the brakes by repeatedly flooring them from high speeds. After a severe enough overheating event, the pad material will glaze over and fail to grip the brake rotor. The same thing happens after overheating a clutch through excessive slippage. If you've ever gotten the clutch hot enough to smoke, then odds are that you've permanently fried it.
The pressure plate springs that hold the clutch disc to the flywheel are just like any other springs in your car -- they will eventually soften through repeated heating and cooling cycles and loose a certain amount of their holding power. This usually takes quite some time -- odds are that you'll wear out the friction material at least twice as fast as the plate springs -- but it is part of normal wear and tear. Spring breakage, on the other hand, isn't as common but does happen.
Scored and grooved flywheels typically don't happen unless the driver has knowingly allowed the clutch to slip long after the friction material has bid adieu. Machining the flywheel is an option if the grooves aren't too deep, but replacement of the flywheel is mandatory if they are. Don't allow a grooved flywheel to run unchecked; those grooves can create stress risers in the flywheel, which precipitate cracks and a bomb-like flywheel explosion about 6 inches from your right foot.
