What does a Thermostat Do?

A thermostat allows something to turn on or off at a set temperature.

A thermostat can be an electrical switch.
Examples are a room thermostat that controls the temperature of a room
and a cylinder thermostat that controls the temperature of a hot water cylinder.
These thermostats form part of the heating controls of a central heating system.
An immersion heater also has a thermostat which is an electrical switch.

A thermostat can be a mechanical device that controls the flow of water through a valve.
Examples are thermostatic radiator valves (TRVs) fitted to radiators
and thermostatic mixer valves fitted to showers or hot water taps.