Constructive interference: Constructive interference is when two sinusoidal waves, having the same displacement, exactly overlap each other to form one big wave (that is double the amplitude). It only occurs when both waves are displaced upward or both are displaced downward.
Destructive interference: Destructive interference is when two interfering sinusoidal waves are displaced oppositely (one up and one down). When they have the same displacement, one wave has a positive amplitude and the other has a negative one. The resulting displacement of the two waves is balanced.