Which is the normal sign of the electric charge among the two sides of the neuron plasma membrane? What is the potential difference (voltage) generated among these two sides? What is that voltage called?

As in most cells the region just outside the surface of the neuron plasma membrane presents a positive electrical charge in relation to the region just inside that therefore is negative.

The normal (at rest) potential dissimilarity across the neuron membrane is about -70 mV (millivolts). This voltage is known as the resting potential of the neuron.