Voltage has more than one name in electrical texts: it's sometimes referred to as emf (electromotive force). Tesla's contemporaries often used the term tension, or it may be called simply potential. The last two, tension and potential, underscore the relative nature of electrical force: a voltage is strictly a measure of charge difference between two points. Though we may speak of a 12-volt car battery, it is with the understanding that twelve volts is measured between this battery's positive and negative terminals. Between the negative terminal of this battery and the negative terminal of another, identical battery, there exists no potential. There is no electrical field, no difference in charge that would impel electrons to move from one negative terminal to the other.
Between the negative terminals of a 12-volt battery and, say, a 3-volt battery there is, however, a difference of 9 volts. In other words, the smaller battery's negative end actually looks nine volts positive to the negative side of the 12-volt battery; electrons will flow to the 3-volt battery. Voltage, thus, is always a relative quantity, the electrical attraction that exists between two specific points.
We often take some convenient reference as a zero voltage, perhaps the earth, and assign positive and negative values according to potentials we measure relative to this "ground." This means, of course, that when a megavolt cloud empties some of its charge as a lightning bolt, the earth is taken as zero and the cloud is reckoned to have a negative voltage. (Rather confusingly, though, and because of an arbitrary sign convention, texts in electromagnetic theory speak technically of a current as flowing always from positive to negative, whereas the actual, physical state of affairs is a motion of electrons from negative to positive. Everyday usage usually intends that electrons and "current" move in the same direction.)