Three-phase power refers to an electrical system that has three voltage or current curves. While we are used to thinking of electric voltage as being constant (for instance, receiving 120V service) in reality the voltage of an electric line is continually fluctuating from positive to negative values. A three-phase system delivers power along three wires, with each wire having its own voltage curve.
A three-phase transmission or distribution line requires three or four wires. In the case of a four-wire system, the system includes three-phase lines plus a neutral.
Three-phase power is produced by all large-scale generating units and is what flows through all transmission lines. Many distribution lines are also three-phase while others are single-phase. Three-phase distribution lines are commonly used as feeders from which single-phase lines are tapped off. In many cases, three-phase distribution lines also directly serve customers. Three-phase power is most commonly used by customers with larger loads and especially in cases where there is significant load from motors.
Three-phase power is generated by spinning a magnet inside three separate independent coils of wire. Each phase wire of a three-phase distribution line is connected to one of the coils.
The three separate independent coils of wire produce three separate independent voltages with different timing, as shown here. Notice that there are three single-phases of power: phase A shown by the blue line, phase B shown by the green line, and phase C by the red line. Phases A, B, and C are mirror images of each other, except that their time sequence is staggered. So when phase A is going through zero, phases B and C are not. Likewise, when phase A is at its maximum value, in either direction, phases B and C are not.