Water in your engine

My brother's car has problems with the cooling system in the engine but how many of you know how the whole cooling thing works? What's the difference between the thermostat and the fan switch? What is the radiator for and why isn't it always hot?
As you might imagine, water is used to keep the engine block cool since all those miniature explosions cause heat build-up which if uncontrolled would quickly lead to the engine going out of shape and things going bang. Something that is important however is that we don't want the engine to be too cold either. Too cold and the oil does not flow well and also the engine parts are not at design temperature causing leaking of fluids around pistons which makes the engine less efficient.
The block is simply not able to regulate its temperature. Although you could design it well for moving, if you were stuck in traffic, there would be little moving air and quick failure of the engine.
OK, if we add water to the engine and make it circulate via a radiator, this allows us to cool the engine. We need a pump - called the water pump - to be fixed directly to the engine so that it always turns when the engine turns. It doesn't have to be very big and will probably be fitted directly to the engine block where the large rubber hose is. This will pump the water to the top of the radiator where it will naturally cool and fall through the radiator where it will return to the engine block. As mentioned before, in traffic where there is a lot of heat build up and little cooling, a fan-switch in the side of the radiator will switch on and cause the fan to pull larger amounts of air across the radiator causing it to cool quickly. Some cars have more than one fan and more than one switch. The fan should always be able to keep the engine cool (well 80-90 degrees!) even when staionary.
Remember how we also don't want the engine too cold? Well there is another gadget in the circuit called a thermostat. This remains closed when the engine water is cool and ensures the water in the engine does not go to the radiator, it just circulates round the engine. This allows the engine to warm up as quickly as possible without cooling it right from the word "go". Once the water gets hotter (because the engine has warmed up) the thermostat opens and allows the water into the radiator.
OK so how can we fault-find?

1. Please don't burn yourself! The engine can get to around 100 degrees C or more if the cooling is not working properly.
   
2. Does the water system have enough water in it? There should be a small expansion tank under the bonnet with min and max marks on it (and dirty looking water). You need some space to allow for expansion and steam build-up so don't overfill it.
3. If your car overheats, does the hose to the radiator and the radiator itself get hot? If neither then the thermostat might be broken or seized. If the hose is hot but not the radiator then the thermostat is open and possibly the water pump itself is not working.
4. If the radiator is hot but you are still overheating then perhaps the fan-switch (or less commonly the fan itself) has broken/fuse blown. You can check this with a multi-meter if you have basic electrical skills.
5. If all is hot and the fan is switching in but you are still overheating then there might be a blockage in the water system, especially if someone has used "Rad weld" incorrectly to attempt to fix a hole in the radiator. This would cause the water to circulate poorly and therefore the cooling to be less than 100%.
6. Worst case and the garage will have to strip the engine down which is not going to be cheap especially since anything that breaks in the process needs repairing (and things will usually break!) and anything else they find will be recommended for repair (broken piston rings etc).