PID temperature controllers works by adopting P.I.D algorithm and compare the difference between set value and process value, then decide the amount of output should be applied to the load to maintain the process value at or as close as possible to the set value. In a typical field applictaion, a lot of variations contribute to the outcome of the control process.  the PID controller is able to eliminate the impact from the surroundings and be able to adjust the output automatically to acheieve the best outcome. all of Maxwell temperature controller is PID controller, as oppose to PID controller, there are ON/OFF controllers on the market,  PID controller is for system where the tempreature control must be precise and the temperature is indeed critical to the system, like pharmaceutical industry, semi-conductor , some of applications on the other hand is less demanding when it comes to tempreature control precision. ON/OFF controller would be an ideal solution for these applications. almost all PID controllers can be configured as an ON/OFF controller on the site via the software, vise verse is not applicable.  one of our most popular PID controller is LCD display PID temperature controller 

Typical ON/OFF temperature controller is thumb switch setting temperature controller