An ECU - Electronic Control Unit or ECM - Electronic Control Module. An ECU is a type of computer, which takes a variety of inputs from sensors on a diesel engine and uses the information to control the engine actuator. This in turn controls the speed of the engine. An ECU has advantages over a mechanical or electronic governor because:
1) It monitors more of the engines parameters to enable faster response to changes in speed and environmental conditions.
2) It can detect some faults from the engines sensors and shut down the engine to prevent failure.
3) It can communicate with other control elements in the system to provide operator feedback and information. Usually these are provides by diagnostic lights, or by Diagnostic Trouble Codes DTC
Disadvantages would include:
1) Additional cost, ECU's are more expensive than mechanical governors.
2) Software is often required to adjust parameters, which may be limited to the dealer only.
3) Additional wiring is required over a standard mechanical governor.