What defines a real negative voltage?

[Taylor_1989]

This has been bugging me for a long time! Now if electrons are -Q and the V is basically what energy is added to that Q how do you define the voltage as being - or +. For example in a computer system, the data cable has + and - negative voltage, so is the polarity switch at ether end on the cable?

I do apologise if this doesn't make much sense, I am not very good at explaining, what I am trying to ask.

 

[Jony130]

The reference point determines whether we have a positive or negative voltage.
See this example