In metals and certain other materials, negatively charged particles called electrons are free to move around. These materials are called conductors. When a battery is connected across a conductor a potential difference is set up along it and electrons 'flow' down the "potential gradient". However, as electrons are negatively charged, down the potential gradient is from the negative to the positive end of the conductor.
This flow of electronic charge is called a current. It is described as direct current (DC) if it flows continuously in one direction and alternating current (AC) if its direction of flow alternates back and forth.