PID control is the mainstream control method that is required in a lot of industrials, like food processing, plastic extrusion, semi-conductors etc. these industrial often need precision control and overshoot should be limited, PID control is also referred to as “Three-term” control. The three terms are:

P for Proportional
I for Integral
D for Derivative

The output of the controller is the sum of the three factors above, PID control takes different varibles into consideration and calculate an optimized outputs to excecute and dispatch approriate power to the load, therefore controls the process by maintaing the process value as close as it can be to the setting value.

ON/OFF control, is a very simple control method, in a reverse control situation, when process value is lower than the setting value, full power will be applied to the load which is ON status, when process value is higher than the setting value, 0% power will be applied and this is OFF status, if you picture a controller in a system with a valve, the valve will be 100% open so the medium in the pipe will flow through freely under ON status,  the valve will closed completely so there will be no medium flow through in the pipe under OFF status.