Why do we always measure voltages between two points? I would like to know what exactly a voltmeter measures in each point.
 A: The voltmeter does not measure anything at a single point. It measure the potential difference $V$ between two points in the circuit. That's why a voltmeter has two leads, not one.
The voltage at a point depends on what point in the circuit is assigned to be the zero voltage reference point, in theory a purely arbitrary decision, although it is typically the negative terminal of a battery (AC sources have no intrinsic polarity). Then the voltage at a point is with respect to the zero reference point.
Hope this helps.
A: A volt meter measures the difference in voltage between two points, but it doesn't tell you anything about the voltage of individual points.  In fact, by convention, the voltage of the negative terminal on a car battery is arbitrarily assigned the value of 0 volts.  Since the positive terminal of the battery is 12 volts above this, such an arbitrary assignment sets that terminal at an absolute value of 12 volts.
A: Of course the details depend on the voltmeter you are using, but briefly you choose a phaenomenon which depends on the voltage difference between two points and which you can measure, and then derive from that measurement the voltage difference that corresponds to it.
For example: you disguise an amperometer/ammeter as a voltmeter. You know its internal resistance $R$. You measure the current $I$ (number of electrns per seconds times their charge) and then instead of $I$ you output on the display $V=IR$ using Ohm's law. What you actually measured is a current, but you turn it into a voltage knowing that the relationship $V=IR$ exists.
You can use several other $V$-dependent effects, including thermal dilation, magnetic induction and many more. But the principle is always to measure something and derive the voltage difference that caused what you measured. Each instrument works best in a given setting and depending on how big the voltage you are measuring is.
A: Technically, grounding (provide 0 V). But in physics, relative value is more useful than absolute value in most of time. Ruler may be your very first tool to learn and use in measurement, it just provides you a length relative to man's feet, you just use the relative data to compare which not only make sense but also convenient.
