I'm guessing that you are a reletivly new member, so welcome!
The ECU (Engine Control Unit) or PCM (Powertrain Control Unit) or EEC (Electronic Engine Control) are all the same thing. They refer to the car's main computer system which contains a flash prom on EECIV and above which contains values and code for controlling all the car's electronic equipment.. most importantly the car's fuel injection, automatic transmission, and engine control curcuits.
This part of the forum is all about tuning, or specifically changine parameters in the flash rom to maximize the car to run a specific way. From the factory, most "family" cars are tuned for ride comfort and fuel efficiency, not performance. By tuning this code, you can run higher octane fuel efficiently, improve shifting performance, and even configure the car to run more extreme modifications such as larger injectors, nitrous, or even forced induction (supercharger or turbocharger).