One phase,two phase and three phase
One phase or single-phase has a two-wire output. Three-phase has a three wire output with equal voltages between any two two wires. There is a 120 degree phase displacement between each of the three output waveforms. Two-phase was used early in the history of electric power but mostly abandoned in favor of three phase. It has two outputs with equal voltages displaced by 90 degrees from each other. It was implemented with three or four wires. There is some erroneous use of the term "two-phase" to describe the single-phase, three-wire, dual-voltage system used in the USA, Canada and perhaps a few other places.