Two-conductor or four-conductor wire CAN bus
The minimum amount of signals used in CAN are 3: CAN High, CAN Low and signal ground. Fundamental data communication of any kind dictates that you absolutely must have a signal ground, thus the 2 conductor cable is hobbyist nonsense. 

There is a whole lot of broken, amateur CAN implementations on the market, where they share the supply ground with CAN signal ground. This is a no-go in the kind of applications where you usually find CAN, namely automotive or industrial. Those kind of applications tend to have very high currents and noise flowing through the supply ground. (In this context, "very high" is 1A or more.)

Optionally, you can use a shield - not to be used instead of signal ground. Especially suitable for higher baudrates and also when longer cables are needed.

If you use galvanically isolated CAN, you also need to provide power for the secondary side, to supply transceivers and isolator electronics. In this case the signal ground is the ground for the secondary side. 
No, "CAN-PWR" refers to supply for a galvanically isolated CAN bus. "CAN-GND" refers to the mandatory signal ground.
A missing signal ground might cause all kinds of intermittent errors and jeopardize the reliability of the whole CAN bus.