When a violin string is bowed, the string vibrates as a whole.  The frequency of the string’s vibration is its fundamental frequency, which in general could be described as being equal to f hz.  However, at the same time the string will also vibrate in a series of parts (Seashore, 1938/1967).  It vibrates in halves, producing the first harmonic or overtone, which will have a frequency of 2f hz.   It also vibrates in thirds, producing the second harmonic which will have a frequency of 3f hz.  This pattern repeats itself, where a harmonic will be a wave that has a frequency that is an integer multiple of the fundamental frequency.  Harmonics are also called overtones, as well as partials.  The timbre of a sound source is the result of different weightings (i.e. different amplitudes or different amounts of energy) associated with the sound source’s harmonic frequencies.

References:

Seashore, C. E. (1938/1967). Psychology of Music. New York,: Dover Publications.