Harmonics are the basic building blocks of periodic sounds. They may also be referred to as partials or overtones, but these words can have different meanings in the same context. Harmonic usually refers to the sine wave components which make up a sound, which are whole-number multiples of the fundamental frequency (lowest tone). Overtones consist of all harmonics except for the fundamental; thus the 1st overtone is actually the 2nd harmonic, and so on. Unlike harmonics, Partials do not need to be related by whole-number multiples, but may instead be completely inharmonic.
Harmonic oscillators such as a bowed violin or the human voice contains a nearly-infinite amount of harmonics, starting with 1f, 2f, 3f, 4f... where f is the fundamental frequency. Each of these harmonics has a distinct amplitude, generally decreasing as the 'height' of the harmonic increases. The span between any two of these harmonics is called a just interval.