Direct current (d.c.) is the movement of charge in one direction only. The charge keeps moving the same way round the circuit and never turns back. Cells and batteries supply direct current.
This happens because the terminals of a cell never swap. The positive terminal always stays positive and the negative terminal always stays negative, so the current always flows the same way round a circuit. A battery is two or more cells joined together, and it also supplies d.c.
Many portable devices run on d.c., for example a torch, a toy car or a remote control. If a battery is connected the other way round, the direct current simply flows in the opposite direction, but it is still one direction only.