In automotive electronics, an electronic control unit (ECU), also called a control unit or control module. It is an embedded system that controls one or more of the electrical subsystems in a vehicle whether secondhand or new. Some modern cars even with a reconditioned engine have up to 70 ECUs, including:

Engine Control Unit is also called Powertrain Control Module (PCM), Gearbox / Transmission Control Unit - TCU, Telephone Control Unit - TCU, Man Machine Interface - MMI, Door Control unit, Seat Control Unit, Climate Control Unit.
Managing the increasing complexity of ECUs and number of ECUs in a vehicle has become a key challenge for OEMs. Most modern cars Ford, Volkswagen, Rover etc. are powered by a ECU.

Electronic Control Unit

A control unit is the part of a CPU or other device that directs its operation. The outputs of the unit control the activity of the rest of the device. A control unit can be thought of as a finite state machine.
At one time control units for CPUs were ad-hoc logic, and they were difficult to design. Now they are often implemented as a microprogram that is stored in a control store. Words of the microprogram are selected by a microsequencer and the bits from those words directly control the different parts of the device, including the registers, arithmetic and logic units, instruction registers, buses, and off-chip input/output. In modern computers, each of these subsystems may have its own subsidiary controller, with the control unit acting as a supervisor.
The control unit is the circuitry that controls the flow of information through the processor, and coordinates the activities of the other units within it. In a way, it is the "brain within the brain", as it controls what happens inside the processor, which in turn controls the rest of the PC.
The functions performed by the control unit vary greatly by the internal architecture of the CPU, since the control unit really implements this architecture. On a regular processor that executes x86 instructions natively, the control unit performs the tasks of fetching, decoding, managing execution and then storing results. On a processor with a RISC core the control unit has significantly more work to do. It manages the translation of x86 instructions to RISC micro-instructions, manages scheduling the micro-instructions between the various execution units, and juggles the output from these units to make sure they end up where they are supposed to go. On one of these processors the control unit may be broken into other units (such as a scheduling unit to handle scheduling and a retirement unit to deal with results coming from the pipeline) due to the complexity of the job it must perform.

An Engine Control Unit (ECU)

An Engine Control Unit (ECU) also known as an Engine Control Module (ECM) or Powertrain Control Unit/Module (PCU, PCM) if it controls both an engine and a transmission, is an electronic control unit which controls various aspects of an internal combustion engine's operation. The most simple ECUs simply control the quantity of fuel injected into each cylinder each engine cycle. More advanced ECUs found on most modern cars also control the ignition timing, Variable Valve Timing (VVT), the level of boost maintained by the turbocharger (in turbocharged cars), and control other peripherals.
ECUs determine the quantity of fuel, ignition timing and other parameters by monitoring the engine through sensors. These can include, MAP sensor, throttle position sensor, air temperature sensor, oxygen sensor and many others.
Before ECUs most engine parameters were fixed. The quantity of fuel per cylinder per engine cycle was determined by a carburetor or injector pump.

A Transmission Control Unit

A Transmission Control Unit is a device that controls modern electronic automatic transmissions. A Transmission Control Unit generally uses sensors from the vehicle as well as data provided by the Engine Control Unit to calculate how and when to change gears in the vehicle for optimum performance, fuel economy and shift quality.
Electronic automatic transmissions have been shifting from purely hydromechanical controls to electronic controls since the late 1980’s. Since then, development has been iterative and today designs exist from several stages of electronic automatic transmission control development. Transmission Solenoids are a key component to these control units.
The evolution of the modern automatic transmission and the integration of electronic controls have allowed great progress in recent years. The modern automatic transmission is now able to achieve better fuel economy, reduced engine emissions, greater shift system reliability, improved shift feel, improved shift speed and improved vehicle handling. The immense range of programmability offered by a Transmission Control Unit allows the modern automatic transmission to be used with appropriate transmission characteristics for each application.