Engine Control Module Powertrain Control Module Engine Control Unit
Reading about them online, they seem to be the same thing, computers. So what are the spcific difference between them? Is the other one any better?
From my personal experience with this particular subject you have many variables that come into play. My shop specializes in computer systems on some of the newer vehicles (Mostly Japanese), but we do work on anything given the chance. ECM, PCM, and ECU are generally manufacturer specific. I usually see ECU when referring to Asian vehicle companies. Chrysler likes to use ECM and PCM. Quite a few GM manufacturers use PCM.
If you think about it from the engineers standpoint they can mean different things though.
Engine Control Module would most likely be your control unit dedicated to all electronics/ Sensors dedicated to your engine only. You would probably have additional computers for things like windows, air conditioning, transmission and they all have their names as well.
If your manufacturer uses the term PCM, it will most likely control multiple aspects of your "Powertrain". It perhaps could be that it controls the transmission and engine. Maybe even power steering and emissions as well.
I see that PCM and ECU are the two most used terms for the actual engine computers. ECM is generally a "catch All" term that can mean quite a few things. I have a few friends that are electrical engineers and they use the term ECM for "Electronic control module". Which could literally mean anything driven by a computer.
I guess what it boils down to honestly is what the engineers say it is. I could go on and on, but this is just my personal experience with those terms.
Hope this helped you a little!
The term ECM could (just as @cloudnyn3 says) be used for both Electronic Control Module or Engine Control Module. Different brands has different names for the same stuff, and also same name for different stuff, which can get a little confusing.
The term ECU is often used for Electronic Control Unit (which could be more or less any electronic unit in the car). For example my -97 Range Rover has several ECU's: ECM (Engine Control Module), BeCM (Body electronic Control Module), ABS (for the brakes), Transmission, EAS and some more.
Most of the ECU's are connected with one or more of the other ECU's. Often with a bus system (for example CAN, LIN, K-Line, Flexray, MOST etc). Cars of today often have even more ECU's and larger bus networks.
One ECU can not be said to be better than another (except maybe some quality issues), they just have different domains that they handle in. And they are very seldom interchangeable.
In many cases persons that has spoken to me about "the" ECU/ECM don't really know what they were talking about. But quite often they mean the module that controls the engine. Some say this is because I either don't understand colloqialism or maybe because I'm a pedantic twat. Possibly both. And they might be correct. (they still don't know what they talk about though)