A glaze is pure color mixed with medium that is translucent. If you add any white (or other pigmented color) to the mixture, it is called a scumble.
Glazing is generally used to enrich/enhance a color that lies underneath...a result that you can't get by just mixing wet paint.
Sometimes I lay down a wet glaze into a dry area of my painting in order to begin to paint thick light into it. This not only adds color, it makes it convenient so that I am able to paint "wet into wet"...and yes, that destroys the effect of the glaze.
Before you begin to glaze a color, the paint underneath (sometimes an underpainting) needs to be rather finished looking. Glazing does not build or add form to a painted object....think of a glaze as adding more color and "atmosphere."
And for Pete's sake, use some common sense here, if you put a glaze onto a painting and it does nothing to enhance the look, wipe it off before it is dry and try something else.