What is ECU and how to make the ECU programmer?

Electronic Control Unit, An engine control unit (ECU), most commonly called the powertrain control module (PCM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure the optimum running. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called Look-up tables), and adjusting the engine actuators accordingly.
Automotive ECU is to convert stored procedures chip in simple, replace the programmed chip, as long as open the backplane of the ECU and unplug the original chip and then put the new chip, as some older E-ROM chip only can write just one time, every time you modify the program you must turn the program of original chip into a blank chip and then replace the original chip. If you want to have a program modified by yourself, you need a ecu programmer.