Historically, the transfer of electricity between regions has been over high voltage alternating current (AC) transmission lines (typically along overhead wires and pylons), which means that both the voltage and the current on these lines move in a wave-like pattern along the lines and are continually changing direction. In Europe, this change in direction occurs 50 times per second (defined as 50 hertz [Hz]). The electric power transmitted over AC transmission lines is exactly the same as the power we use every day from AC outlets, but at a much higher voltage.
Unlike an AC transmission line, the voltage and current on a direct current (DC) transmission line are not time varying, meaning they do not change direction as energy is transmitted. DC electricity is the constant, zero-frequency movement of electrons from an area of negative (-) charge to an area of positive (+) charge.
The first commercial electric power system built by Thomas Edison in the late nineteenth century carried DC electricity, but given some early advantages, AC power eventually became the primary power system worldwide. Some of these advantages are no longer applicable (e.g. technology has advanced to allow efficient conversion from AC to DC), and DC transmission is the preferred solution for moving large amounts of power over long distances with considerable social and commercial advantages.