Thermostatic radiator valves are often known by the initials "TRV".
A TRV works by sensing the surrounding air temperature. It progressively closes down the flow through the radiator as the air temperature rises, thereby cooling the radiator and regulating the room temperature.
When fitting a new boiler or replacing an existing boiler there should be a TRV fitted on each radiator in the house except one which is known as the bypass. The bypass can be any radiator in the house but normal practice is to have it as the radiator where the room thermostat is. This will then control the overall temperature of the house.
Minimum building regulation requirements state that TRV's should be fitted in bedrooms.