There are several controls on a central heating boiler - a timer/programmer to tell it when to come on each day - A boiler thermostat to determine how hot the boiler makes the water and an overheat thermostat for emergency shut down if the water gets too hot for some reason.
Outside the boiler there will be a thermostat on the hot water tank to determine the temperature of the tap water.
Most installations also have a Room Thermostat that controls the central heating - my room stat is on the wall in the hall. We keep ours on about 17 because all the heat in the hall escapes upstairs - if the hall is 17 then we can be sure that all other rooms are warmer.
On top of all this some people have thermostats on each radiator for individual control within a room.
Your room stat - if you have one - will look not unlike this picture...
