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For every one joule of fuel energy converted to torque by an internal combustion engine, eight or nine are expended as heat. All of this heat has to leave the engine block, or it will become hot enough that parts of it warp, fuse together, or respond in other ways that will ruin the engine. In an air-cooled engine, fins or other devices are used to sink the heat into the ambient air around the engine. The much much more common liquid-cooled designs rely on liquid coolant circulating through pathways within the engine, and then out to an external heat exchanger, called the radiator.
Actually, that's not strictly true. There's a temperature range that most engines prefer to function in (around 93° Celsius), and the cooling system works to keep the engine near that temperature. The thermostat is a valve that blocks coolant from circulating from the engine into the radiator until the engine is warmed up. Also important is the fan, which blows air across the radiator to push heat into its surroundings. Front-wheel drive cars (and newer rear-wheel) generally have an electric fan controlled by a temperature sensor or computer, whereas older rear-wheel drive cars have a fan powered by the crankshaft driven through a temperature-sensitive viscous clutch. When the temperature becomes too high for these countermeasures to help, the spring-loaded radiator cap is pushed open, and some of the coolant can go into an overflow tank. Once the pressure goes back to a reasonable level, vacuum pressure pulls the coolant back into the radiator.
Think of the radiator itself as a collection of parallel tubes that the coolant passes through, with a couple hundred thin aluminum fins attached to them. Heat moves from the coolant, through the tube walls, and into the fins. Air circulates through the fins, either pushed in as the car moves or pulled in by the fan, and the heat escapes into it. Hot coolant enters the radiator at a top corner, and cooled coolant is pulled by the water pump from the opposite bottom corner. In a car with an automatic transmission, transmission fluid is pushed through another section of the radiator.
Interestingly, a car's heater core is essentially another radiator, but the hot air leaving it enters the car's cabin rather than the outside air. Thus, if your car is overheating and you absolutely must keep driving it, cranking up the heater and rolling down the windows can offload a bit of excess heat.
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