Thermostatic radiator valve (TRV) is a self regulating valve fitted to radiators on one side, to control the temperature of a room by closing gradually as the room gets closer to the set temperature. Older TRVs used wax to adjust as apposed to the modern liquid type which provide a by far quicker adjustment according to the room temperature. The modern valves can also be fitted to either side of a radiator, they are bi-directional.
TRVs shouldn't be fitted in bathrooms as the heat produced from baths and showers can cause the TRVs to close down prematurely allowing condensation to build up rather than preventing it. Also they should not be fitted in the same room as the room thermostat as they can fight each other and either turn the boiler off prematurely or keeping the boiler on when every other room is up to temperature.