There are two types of AC:
A single-phase current is the most common type of current, and thus is usually the configuration delivered by public networks, but also by a single-phase generator. A single-phase AC current is supplied via two lines (phase and neutral), usually with a 220 V voltage difference between them. Plugs can be inserted in both ways.
Because the voltage of a single-phase system reaches a peak value twice in each cycle, the instantaneous power is not constant and is mainly use for lighting and heating but cannot work with industrial motors.
A single-phase load may be powered from a three-phase distribution transformer allowing single-phase lighting to be connected phase-to-neutral and three-phase motors to be connected to all three phases. This eliminates the need of a separate single-phase transformer.
Once Power needs are increased, in the use of large electrical motor for example, constancy and balance pay a key role. Three-phase is the common current configuration for electricity companies, and can also be produced with a three-phase generator. A three-phase current is the combination of three single phase currents.
To carry a given power with 3 separate single-phase cables, 9 wires are needed. To carry the same power in a three-phase cable, only 5 wires are required (3 phase, 1 neutral, 1 ground), which it is why there can be significant savings when properly planning a three-phase current: saving on wires, cables, and also in apparatus using or producing electricity: three phase motor or alternator will be smaller than the same power produced by three single phase equivalent units.