In very simple terms these range of valves help control the amount of hot water that enters and leaves the radiator. By controlling this flow of water, the heat the radiator can generate/output is controlled. Each type of valve will have a range of performance and safety features.
TRV Servicing Tool Insert Remover 10-20mm (DN10-20) - Complete with case, box spanner and replacement seals, for replacing thermostatic inserts without draining off the heating system.