The most basic waveform is a sine wave (sinusoid) (fig. 1). It could be considered the atom of sound. Pure sinusoidal sounds are rare ( tuning forks, drinking glasses being rubbed) and were considered to have strange powers over human behavior at one time.  Most sounds that surround us are of a more complex nature.

This means that inside a sound, that we perceive as being unique, there is a superposition of many sine waves that have, in a way, fused together to become one sound. It's the nature of this superposition itself that determines the resulting waveform (fig. 2) and that is responsible for its timbre. This is called a spectrum.