Most of you who are reading this already have at least a basic knowledge of what vibrato is; that is, the periodic and regular wavering that you hear in many musical instrument sounds including singing voices. But, there are many different types of vibrato, and they are quite different from each other, so let's take a little look at this and see what we can find. Before we even begin, however, let's make sure that we're on the same page regarding two terms which are often used in place of each other, and these are:

Tremolo
  Vibrato

Tremolo is a periodic and regularly-recurring change in the instantaneous volume of a musical tone, and vibrato is a periodic and regularly-recurring change in the instantaneous pitch of a musical tone. One good way to see the difference is to look at a graphical representation of a simple soundwave and see, via Flash animation, the actual effect of tremolo and vibrato on the sound wave. Although these two Flash animations are in the technical article about the Hammond Organ, I've reproduced both of them here for your convenience.