AC and DC
Alternating current flows first one way then the opposite way, then the first way...
Direct current always flows in the same direction, from positive to negative terminal.
Each form of electricity has important advantages.
Alternating current is easily generated by rotating a coil in a magnetic field. Without further processing, the current that emerges is alternating current. The voltage of an alternating source can be easily transformed to higher or lower voltages with the aid of a transformer. Transmitting electricity at high voltages reduces power losses, through voltages are very dangerous, so reducing the voltage at the point of use is important to reduce death and injury.
On the other hand, alternating current cannot easily be stored.
Direct current is slightly harder to generate than alternating current. A slip ring and brushes are used at the output of a generator. Alternating current cannot be transformed up or down into higher and lower voltages. so power losses during transmission are higher. On the other hand, direct current can be easily stored in batteries.