Voltage–along with current, resistance, and time/frequency–is one of the of the key parameters we measure and use to define our circuits and systems. It tells us a lot about what's happening, and it is usually easy to measure: you usually don't even need to break into the circuit, as you need to do with most current and resistance measurements.
But the ubiquity, importance, and ease of measuring voltage also lets us get casual and comfortable with citing it–perhaps too much so, in some cases. There really is no “voltage” at a point, there is only a “potential difference” between two points. In some cases, one of those points is what we call ground or common (click here for my views on that situation) but it doesn't have to be that way. In fact, in some cases, we are more interested in the potential difference between two points, neither of which is at so-called ground or circuit common. This is the case, for example, when we want to measure the output across the terminals of a battery in a series-connected stack of batteries (often called a “floating” measurement).
I have seen technical documentation and annotated schematics which mark the voltage that should be measured at specific points in a circuit, without any note saying with respect to what other point in the circuit these voltages are to be measured. And as any experienced engineer knows, even that innocuous phrase “with respect to ground” can mean any of many difference so-called ground points which actually have a potential difference between them. I actually saw a document once which forgot to mention (since it was so obvious to the designer?), that the voltage at point A was supposed to be measured with respect to some other, non-obvious point B. Good luck figuring that out.
So the lesson is simple: a voltage is really a potential difference, period. And you need to make what those two points are before you start tossing around phrases like “the voltage at this point is X.” That's a simple and easy rule to follow, and one which can save you a lot of frustration and headache. ♦