ECU
From Celica GT-Four / Alltrac - ST165 ST185 and ST205
Available Unit IDs: C01, C02, C03, C04, C05, C06, C07, S01, S02, V01, V02, V03.
Example: <google uid="C01"></google>, <google uid="C01" position="left" ></google>
What is it
The ECU is a computer system that takes information from the various systems and sensors on the car/engine and calculates what fueling and ignition timing is required to best run the engine. Toyota set the standard ECU up to give good power, drivability and long life for the engine and transmission. Those of us who like risk like to play god with the engine and make or buy a new programmable ECU that can push the engine harder.
JDM ECUs tend to have more aggressive timing as they were designed to run on higher octane fuels. Other than this small difference you should be able to swap JDM/USDM/UK of the same year without problems. Some people use this opportunity to put a JDM ECU in their UK or USDM car for more power, so long as they use premium fuels or octane boosters.
The standard ECU is capable of producing error codes to help you understand faults - these should be checked every so often or if you have any fault/symptom.