Amateur Radio Manual/What is Voltage
So far we have learned that valence electrons will flow in a conductor if we provide enough energy or force so that the electrons will leave the atom and jump to the next atom. We can cause a net movement of electrons through a conductor by attaching one end to a negative source and the other to a positive source. Because like charges repel and unlike charges attract, electrons will move from the negative source to the positive end. It just sounds wrong to have something move from negative to positive but remember that negative and positive are charge types not amounts.
The applied force that causes the electrons to flow is called voltage (after the scientist Alessandro Volta) or electromotive force (emf). We give it the symbol or ℰ in equations. Voltage is measured with a voltmeter or multimeter and is the potential difference between two points in a circuit. The basic unit is the volt (V).
Consider a battery. The chemical process in the battery creates a surplus of electrons at the negative terminal and a deficiency of electrons at the positive end (due to the fact that absence of electrons does not automatically indicate presence of positive charge, unless we consider this terminal charged with positive ions). The potential difference or voltage between these two points is the "push" that the battery can supply to a conductor that may connect the two terminals.