EEC=Electronic Engine Control. Ford's name for the entire engine management system. It has evolved over the years EEC-1, EEC-2, EEC-3, EEC-4, and EEC-5.
PCM=Powertrain Control Module. The processor (computer that controls the powertrain, that is the engine and transmission.) It is the module only, not the wiring and sensors.
ECU=Electronic Control Unit. The older name for the PCM, however it controlled the engine only, not the transmission.