Emzy,
Sounds like you're new to this so here's a short "teach-in".
The clutch connects the engine power to the gearbox. There is a thick circular metal plate bolted to the engine crankshaft. This is the flywheel. It has a ring of teeth round its edge, used by the starter motor to crank the engine.
Whenever the engine is running, the flywheel is therefore spinning. It's called a flywheel because it has some mass and this helps smooth out the rotation of the engine, much as a flywheel used to do for steam powered engines.
The clutch is fixed to the flywheel and has two main components. First there is another thick circular metal plate that is pushed towards the flywheel by strong springs. This is referred to as a pressure plate. It rotates with the flywheel.
Sandwiched between the flywheel and the pressure plate is a disc with friction material on both sides. This is called the driven plate or clutch plate. The friction material is similar to that on brake pads/shoes and that's what smells when it gets too hot and burns. The centre of the driven plate has a splined hole. This slides over a splined shaft sticking out of the gearbox. This is the gearbox input shaft.
When clutch is engaged (foot off pedal), the driven plate is gripped tightly between the two steel plates, under spring pressure. The power from the engine is therefore transferred to the gearbox and, if a gear had been selected, the road wheels turn, otherwise the gearbox input shaft just spins without any further power transmission (hence you can rev the engine in neutral).
When the clutch is disengaged by pressing the pedal, foot pressure operates the clutch master cylinder, hydraulic pressure transfers the movement to a piston in the clutch slave cylinder and a fork presses on the clutch release bearing (sometimes called the thrust bearing) which operates on the spring mechanism of the pressure plate. The pressure is now relieved, the flywheel and pressure plate continue to spin, but the drive chain, from driven plate, through the gearbox and differential to the road wheels is no longer powered. This allows gears to be changed or the speed to be reduced or stopped, without any effect from the engine.
When releasing the clutch, the driver's careful control of the pedal position ensures that the springs of the pressure plate gradually apply pressure to grip the driven plate and once again establish full power transmission to the road wheels via the gearbox.
Clutch problems include (in no particular order):
1. Driven plate linings worn, clutch slips, quick test is accelerate hard up hill in second gear. If suspect, the clutch will slip, engine revs will increase without road speed increasing.
2. Pressure plate worn/damaged. The release mechanism consists of spring steel fingers that get worn or cut off. Often associated with strange clicking noises.
3.Release bearing worn/damaged. The bearing referred to above is between the clutch release fork (non-rotating) and the pressure plate fingers (rotating). This small bearing does a lot of work and is prone to seizure or other failure. It it generally renewed whenever the other clutch parts are renewed. Faulty release bearings often make whining or rattling noises. The noise will be different (or stop) when the clutch pedal is depressed.
4. Hydraulic problems. The hydraulic system is quite basic: master cylinder, pipe, slave cylinder. However, the slave cylinder is in a hostile area and prone to damage of the rubber seals or ingress of debris. The hydraulic oil is shared with the braking system so it pays to keep an eye on the reservoir. The oil is an effective paint stripper so caution with spills/leakages. It is also hygroscopic (absorbs water), which is why manufacturers recommend periodic renewal and why it's not a good idea to put old oil back in the system.
5. Driving style. Rule is foot away from pedal except when changing gear. Resting the foot on the pedal ("riding the clutch") keeps some pressure on the release bearing, promoting its early failure.
Now, on to clutch renewal. As mentioned by a previous poster, you can pick up a three piece clutch kit for around £50. I've done Mk1 and Mk2 and price was about the same. The three pieces are driven plate, friction plate and release bearing (you know what these are now don't you!). You will probably entrust the job to a garage but if you want to know what's involved, see my guide on this forum.
red