Q:

What is the COM terminal on a DC Power Supply?

We have a HP E3631A power supply at my university's lab. There are 6 ports. My questions are as follows. 
What is the green port titled 240VDC Max? I was explained to an extent that it was the ground associated with the ground of the building. I'm just not understanding it completely and why there is such an option. 
Under the 25V option there is a black port titled "COM" between the two red ports. Why is it titled COM? What does this mean? I know this is the port I ground to everyday when I use this but don't know why it's called "COM". 
Under the 25V option there are two red ports and the black port titled "COM" as I mentioned above. I'm confused why one of the red ports is titled "-". Does this have something to do with polarity? Could someone explain the definition or use behind this red port titled "-"? 
Thanks for any help.

A:

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".

