Voltmeter measure the voltage difference between two places, such as two points within a circuit . The typical voltmeter has two terminals, connected to wires or "leads." Place the tip of one lead at one of the points to be tested and the tip of the other at the other point, and the meter shows you the difference.
People use the word "voltage" all the time without really understanding it. Voltage is not current -- that is, it's not the flow of electrons from Point A to Point B. Rather, it's a measure of the ability to deliver an electric charge from Point A and Point B. Think of it like a fire hose: Current is the water flowing through the hose, while voltage is the pressure behind the water that causes it to flow. If there's no voltage, there will be no current. Voltage is by definition a measure of difference: the difference in electric potential between Points A and B. That's what the voltmeter measures.
In an analog voltmeter, each lead