A wave is a disturbance in the equilibrium conditions of a medium that propagates through the medium with a speed V that is dependent on the properties of that medium. The wavelength the distance traveled through space by a wave disturbance during a time equal to one period of the source creating the wave. Waves are said to interfere whenever tow, or more, wave disturbances travel through the same space at the same time. Waves must be of the same type in order to interfere. Constructive interference occurs when the wave disturbances add to produce a bigger wave.
Destructive interference occurs when the wave disturbances subtract to produce a smaller wave.
A system is said to exhibit resonance when the amplitude of the motion of the system is the largest motion possible. Resonant standing waves on a string occur when the length of the string is equal to an integral number of half wavelengths. A system can resonate at many frequencies. The lowest frequency at which a system can resonate is called the fundamental frequency. Resonant frequencies higher than the fundamental frequency are called overtones.