A temperature controller is an device that is used to control temperature. It does this by first measuring the temperature (process variable), it then compars it to the desired value (set value). The difference between these values is known as the error (Deviation). Temperature controllers use this error to decide how much heating or cooling is required to bring the process temperature back to the desired value. Once this calculation is complete the controller will produce an output signal that effects the change required. This output signal is known as the (manipulated value) and is normally connected to a heater, control valve, fan or some othe "final control element" which actually injects or removes heat from the process.
Temperature controllers form one of the four parts of a temperature controlled system. To help visualize this we will consider an oven. The four parts would be :
1 The oven
2. The heater
3. The thermometer (or thermocouple)
4. The controller