Electricity flows in two ways, either in alternating current (AC) or in direct current (DC). The word electricity comes from the fact that current is nothing more than electrons moving along a conductor, like a wire, that have been harnessed for energy. The difference between AC and DC has to do with the direction in which the electrons flow.
In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forwards" and then going "backwards." The power that comes from wall outlets is AC.