All waves, if travelling through ahomogeneous environment where their speed is constant, are composed of pure sinusoidal functions with the following form:
$${\psi(x) = A \sin\bigl(2\pi(kx \pm ft)\bigr) }$$This function describes the amplitude of a wave as a function of space, x. A describes the maximum amplitude of the wave, k is the wavenumber, f is the frequency, and t is the time.
The aplitudes of waves occupying the same space add together to form new waves. This property is called superposition. If the amplitudes of the waves at a given position have the same sign, the amplitude of the combined wave increases. This is called constructive interference. If the amplitudes of the waves at a given position have the opposite sign, the amplitude of the combined wave decreases. This is called destructive interference.