7. Harmonic Structure
All sounds (except a sine wave) contain multiple frequencies called overtones. If these overtones sound musical, they are called harmonics or partials. It is the combination of harmonics and their partials that give sound its tonal characteristics i.e. whether it sounds like a piano or a flute etc. This tonal characteristic is known as the timbre, hence the term multi-timbral that describes a synthesizer capable of playing more than one sound at once.
A harmonic is a direct multiple of what is known as the fundamental frequency. The "Fundamental" is the lowest frequency within a sound and the one which determines its pitch. So if a full orchestra was playing Concert A, the lowest frequency in each instrument would be 440hz. However each instrument would contain different multiple frequencies at different levels.
With most musical sounds, the fundamental is of a greater level than the harmonics. This is what makes them easier to pitch. Unmusical sounds like machine noise have overtones that are louder than the fundamental which is why you could find it difficult to determine what musical note an engine makes. Most drum sounds are also difficult to pitch for the same reason which is why the musical tuning of a drum is not as critical as with other instruments.
Fundamentals determine Pitch
Overtones or Harmonics determine Timbre