In simple terms a heat source will distribute heat around a building using a network of pipes connected to heat emitters. In the UK most homes use a gas boiler connected to a series of radiators distributed throughout the house to create an even temperature in the building.
The boiler will burn the gas which in turn heats water inside a heat exchanger inside a boiler. The pump (sometimes inside the boiler sometimes outside the boiler depending on the type of boiler) will then push this heated water through a ‘flow’ pipe to the radiators.
The water will then pass through the radiators and give off its heat to the air around the radiator and then pass on through the ‘return’ pipe back to the boiler. This process happens continuously until the area reaches the correct temperature then your system control will tell your boiler to stop firing and after a short period the pump will also stop running.
Most of the heat a radiator emits is through convection and not, in fact, through radiation as the name would suggest. This means that the air above the radiator is warmed and as warm air rises is sucks cooler air from below to form a convection current. If you hold your hand above a hot radiator you can actually feel the air rising from it.
To control your heating you should, as a minimum, have time and temperature controls. This means having a thermostat positioned somewhere sensible (not above a radiator or in direct sunlight) to detect the ambient room temperature so that once that temperature has been achieved it can switch off the heating thereby saving energy. It is also a good idea to be able to set on and off times for your heating cycles.
For more precise control in each of the rooms it is good practice to install thermostatic radiator valves which can control temperature in individual rooms. It is also possible to ‘zone’ your heating so you can, for example, set different temperatures upstairs and downstairs in your house.