Electricity is defined as the flow of electric charge, which, in some metals, is carried by electrons that are able to jump from atom to atom under a sufficiently strong electric field.

The types of metals that are able to easily conduct electricity are those whose atoms have extra electrons, not necessary for binding the atoms together. When an electric field or potential is applied by way of a voltage source, the extra electrons flow toward the positively charged end of the voltage source.

Since the positive flow of electric current is defined as the flow of positive charge, and since electricity in metals is carried by negatively charged electrons, the flow of electricity in meatls is, oddly, defined to be in the opposite direction than the flow of electrons. That is, even though a battery causes electric current to flow into a metal wire out of its positive terminal, the postitive current is actually carried by negative electrons flowing in the opposite direction.