What a room thermostat, TRV, programmer and heating zone each control, and how they cut wasted heat.
Room thermostat
A room thermostat senses the air temperature in one representative space and switches the whole heating circuit off once that temperature is reached, calling for heat again as the room cools. It controls the system as a whole, not any single radiator.
Thermostatic radiator valve (TRV)
A TRV is fitted to an individual radiator and throttles the flow through that radiator alone, responding to the air temperature around its own sensing head. That gives room-by-room control: a room that reaches its set temperature has its radiator throttled back while the rest of the system carries on. A TRV has no electrical link to the boiler, so closing it does not itself end the boiler's firing demand.
Programmer or timer
A programmer (or time switch) sets the periods when the heating and hot water are allowed to come on, independently of temperature. Thermostats then decide whether heat is actually needed within those periods, so together the programmer and the thermostats give both time control and temperature control.
Zoning
Splitting a property into separate heating zones, each with its own thermostat and controls, lets areas that are not in use stay unheated while occupied areas are kept warm. This is different from balancing, which shares flow between radiators on the same circuit rather than switching whole areas on or off.