Interference is the result of two or more waves meating each other at a praticular point in space at the same time. The net waves that forms at the point of interference has an amplitude that is equal to the vector sum of all wave amplitudes at this point. This concept of the sum of all amplitudes is called the principle of the superposition.
When two wave pulses interfere with each other they can form pulses of larger amplitude, or, if they have oposite orientation they can destroy each other. The increase of the waves amplitude is called constructive interference and the decrease of the amplitude is called destructive interference. The destructive and constructive intereference are shown in the diagrams below.
The Tacoma Narrows Bridge (USA) collapsed in 1940 because the winds blowing across it matched its natural resonant frequency.
