They can be "dedicated" systems that perform a set of pre-programmed instructions, or computer controlled ("dynamic") systems that react to input from sensors.
Most control systems use feedback loops to tell them how to behave. This is when information from a sensor (e.g. temperature sensor) is used to decide how to use an output device (e.g. heater).
The output device will have some sort of effect on the next reading from the sensor - e.g. switching on the heater will raise the temperature at the temperature sensor.