All ECU's basically do the same thing. They are all programmed to to adjust fuel and ignition based on engine sensors. The sensors generally will be Crank angle, throttle angle, Lambda, manifold pressure, coolant temp, intake air temp, plus optional knock and cam. Some have leaning functions that can be used during the initial tuning then turned off. Ecu's can generally be split into 3 groups. Low, medium and high end. The high end ECU's are much quicker and have the ability to control spark per cycle and cylinder whereas the low and medium groups are just not fast enough to do this. The problem I have found with the high end stuff is they are just too complicated for most of us to get to grips with so tuning costs can be very expensive.