An electrical contractor in Austin, TX, posted a super-informative blog post about understanding AC and DC electrical currents. This is what the differences boil down to: "DC" means "direct current". DC current moves in one direction. It is typically used in situations where smaller amounts of power are required, like in small, battery-operated devices. "AC" means "alternating current". With alternating current, the current can move in more than one direction at a time. AC is our standard electricity in the USA. Since AC current is high power, we need transformers to transform the electricity from its high-powered state to a level that is safe to use in our homes.