ECU

An (ECU)  engine control unit, or as it is called after the 1980s as the power-train control module (PCM),is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running. It is like a brain , it controls all the sensors to make the vechile run smoothly.

Stock ECU vs. Moddified and VTEC vs, Non VTEC ECU Differences

http://www.youtube.com/watch?v=5tobaf_jk90