What is the COM terminal on a DC Power Supply?
The green port is probably connected to the grounding pin on the AC cord that connects this supply to the wall, which is probably connected to a copper rod stuck in the Earth somewhere near your building, like this:



Read more at Wikipedia.

"COM" means "common". The three connectors on the right, the ones labeled "+/-25V", are relative to this. If you were to measure the voltage between the two red terminals, it would be 50V; "COM" gives you a voltage in the middle. If you call "COM" 0V, then the other two are +25V and -25V.

The black port of the group labeled "6V" is simply the more negative of the two ports, and that's why it has a label "-". Just as a battery has a "positive" and a "negative" end, so too does this power supply.

The important thing to understand here is that voltages are always relative. There is no "absolute voltage". When someone says "six volts", he means "six volts more than some other thing". What the "other thing" is depends on context and convention. Usually it's whatever has a ground symbol on it in the schematic. Voltages can be above or below this, or there can be no direct connection, in which case they are said to be "floating".