Different manufacturers call it different things. Whether you call it ECM, BCM, PCM, TCM, or something else, they all mean pretty much the same thing. It's your car's computer. Computers hold information that can be changed in a way that affects the performance of the vehicle. You can change just about any part of the vehicle performance from gas mileage to how the transmission shifts.
Of course, the time when you could do these modifications yourself is long past. Now it requires specialized equipment and training. Although the cost varies depending on what you want done, it is very affordable and it's mobile. You don't even have to bring your car to me. I can go to you and do this in your driveway.
Service limited to Ford, GM, and Chrysler.