Harmonics (overtones) 
A difference is noticed between sounds is that some sounds are pleasant while others are unpleasant. A beginning violin player sounds very different than a violin player in a symphony, even if they are playing the same note. A violin also sounds different than a flute playing the same pitch. This is because they have a different tone, or sound quality. When a source vibrates, it actually vibrates with many frequencies at the same time. Each of those frequencies produces a wave. Sound quality depends on the combination of different frequencies of sound waves.
Imagine a guitar string tightly stretched. If we strum it, the energy from our finger is transferred to the string, causing it to vibrate. When the whole string vibrates, we hear the lowest pitch. This pitch is called the fundamental. Remember, the fundamental is really only one of many pitches that the string is producing. Parts of the string vibrating at frequencies higher than the fundamental are called overtones, while those vibrating in whole number multiples of the fundamental are called harmonics. A frequency of two times the fundamental will sound one octave higher and is called the second harmonic. A frequency four times the fundamental will sound two octaves higher and is called the fourth harmonic. Because the fundamental is one times itself, it is also called the first harmonic.
When an object vibrates it propagates sound waves of a certain frequency. This frequency, in turn, sets in motion frequency waves called harmonics.
The basic frequency and its resultant harmonics determine the timbre of a sound. The greater the number of harmonics, the more interesting is the sound that is produced.
It is an object's ability to vibrate and set up harmonics that determines the pleasantness of the resultant sound. Crystal glass set up harmonics that are more pleasant than harmonics of ordinary glass.
The combination of fundamental frequency and its harmonics is a complex wave form.