Wave superposition is the principle that when two or more waves overlap in space, the resulting wave at any point is obtained by adding the individual wave displacements at that point.
The displacement will depend on the relative position and amplitude of the waves combined together.
Wave superposition is only possible when waves are of the same type (for example: electromagnetic wave cannot be combined with sound wave).
Interference is a phenomenon that occurs when two or more waves overlap.
When the amplitud of the waves is added and the resulting amplitude of the combined wave is greater than before, is called Constructive Interference.
When the amplitud of the waves is added and the resulting amplitude of the combined wave is lower than before, is called Destructive Interference.
In the figure above, the crests and the troughs of the two waves are aligned. In this case, a constructive interference occurs and the resulting wave has a greater amplitude.
Likewise, in the figure above, the crests and the troughs of the two waves are not aligned. Instead, the crests of one wave are aligned with the troughs of the other wave. In this case, a destructive interference occurs and the resulting wave has a lower amplitude.
As for the subtraction of waves, exactly the same process occurs but in the opposite direction (see figures below).
