A thermostatic radiator valve measures the room temperature and regulates the radiator temperature accordingly. This means the radiator will automatially maintain your desired temperature. As the room temperature rises a thermostat in the centre of the valve handle expands and shuts off the valve mechanism when the desired temperature is reached. The desired temperature is set by turning the valve handle.
Thermostatic radiator valves are recommended by the Energy Saving Trust and under the government's efficient heating advice.
Note that the term thermoregulating valve is also used to mean thermostatic radiator valve. Strictly speaking a thermoregulating valve has both hot and cold supplies, such as on a shower, but more and more the term has come to have both meanings.
Manual radiator valves have no thermostatic control. The radiator temperature is controlled through the thermostat on your central heating system and the manual valve is only used to turn the radiator on and off. The maximum flow of hot water out of the radiator is controlled by adjusting the lockshield valve on the radiator outlet as for thermostatic valves.