The voice does not hold steadily and strictly to the pitch, and according to the amount of the variation a corresponding vibrato, or tremolo, is produced.
The action of stringed instruments illustrates this statement.

The finger of the violinist vibrates on the string by rocking rapidly back and forth and the vibrato is the result.
The same is true of the human instrument.

By variation of the tension, the vocal apparatus sends forth several tones in alternation, of a slightly different pitch, which together produce the effect.
Three sources are ascribed for the vibrato; one is a rapid, spasmodic vibration of the diaphragm, causing variation of breath pressure; another is the alternate tension and relaxation of the larynx and vocal cords; a third is that commonest of faults--throat stiffness.
Either cause is possible, and variation in the pitch or intensity of the tone is the result.

Sufficient investigations have not been made to make the matter certain, but tremolo, trembling of the vocal organs, and muscular stiffness, or unnatural tension, seem to go together.
It is quite possible in the early stages of culture so to train the voice as to use the vibrato or not at will, but if not early controlled this, like other bad habits, gains the mastery.

Excessive vibrato has spoiled many good voices.