Timbre is a descriptive word used to help describe the 'color' and envelope of a sound.
All sound is made up of simultaneous sounding tones. In the 1700 Joseph Fourier a French mathematical physicist proved that all sound can be synthesized be adding sine waves. The way these sine waves are added together make things sound different.
Different sound don't sound the same due to different fundamental frequencies, harmonics, complexity, and envelope.
Timbre is made up of Harmonics and Envelope. The Harmonics define the different frequencies present in a a sound and the envelope defines the amplitude through time.
http://www.rane.com/par-t.htm Harmonics defined @ rane
Understanding Harmonics Fundamental Frequency and Harmonics Instruments
* Guitar string * Open-End Air Columns * Closed-End Air Columns
Natural harmonics are multiple of the fundamental/
Understanding Harmonics @ Harmony Central