A phase is the current or voltage between an existing wire and a neutral wire. A phase is defined by the electric load distribution corresponding to the type of unit, which could be a single-phase or a three-phase electrical system.

Three-phase power is typically used in commercial/industrial situations and large homes with multiple large electric appliances drawing larger currents of electricity. If your property is drawing a lot of power, three-phase power would be installed to avoid power fluctuation.

Single-phase is a service configuration offered by electric utilities and is typically provided to residential and small commercial customers. It is most often provided as 120/240V service, 120 volts for the lights and the outlets and 240 volts for the large energy-using appliances.

Single phase is commonly called “residential voltage” for many reasons; one of which, is that most homes use it (it's the power present in wall plugs).