Phase difference:

Two alternating current waves may have exactly the same frequency, but they can have different effects because they are out of sync with each other. This is true when alternating current waves are added together to generate a 3rd, or composite, signal.

If two alternating current waves have the same frequency and magnitude, but differ in phase by 180 degrees, they will cancel each other out, and net signal will be zero. If the 2 waves are in phase, the resulting signal will have same frequency, but twice the amplitude of the signal alone.

If two alternating current waves have the same frequency but different magnitudes, and differ in the phase by 180 degrees, the resultant composite signal will have the same frequency as the originals, and the magnitude equal to difference between two. If 2 such waves are in phase, the composite will have the same frequency as the originals, and a magnitude equal to the sum of two.

If the waves have same frequency but differ in phase by some odd amount like 75 degrees or 310 degrees, the resulting signal will have same frequency, but will not have same waveshape as either of the original signals. The range of such cases is infinite.

Household utility current, as you obtain it from wall outlets, consists of a 60-Hz sine wave with one phase component. But energy can be transmitted over long distances in 3 phases, each differing by 120 degrees or 1/3 cycle. This is what is meant by 3-phase alternating current. Each of the three alternating current waves carries 1/3 of the total power in the utility transmission line.