Antifreeze and the cooling system. This is probably the most stressed system in the modern car. It is not only responsible for keeping the engine cool, but provides heat into the cabin, keeps the water from freezing or boiling as well as provides lubrication for the water pump.
Water is great at transferring heat and oxidizing metal components. Aluminum can and will 'rust' just as cast iron will so it's important to provide some type of corrosion inhibitor. The pictures above show the end result of running straight water in a cooling system. The top one shows massive rust accumulation and the bottom shows the water pump impeller completely disintegrated!
When properly mixed, the antifreeze changes the boiling and freezing points of water. We all know that at sea level, water will freeze at 32*F and boil at 212*F, but with a mixed 50/50 with the antifreeze, those properties change to -40*F and about 225*F. The trouble is, antifreeze alone isn't a good conductor of heat which is why it is mixed with water to get the best of both properties.
Another way we get more protection from boiling is by using a radiator cap that holds in some pressure. As the system heats up, pressure builds due to heat expansion. By maintaining pressure on the system, we can further up the boiling point of the coolant. If the coolant boils, it can't transfer any heat away from the engine to the radiator and overheating will soon follow. A 15psi cap can up the boiling point to as much as 265*F!