Different paint colors have different degrees of transparency and opaqueness. In other words, some paints simply cover better than others. For example, pink and yellow-oxide are more opaque colors than dioxazine purple or ivory black. Opacity really depends on many factors, such as quality of paint, as well as the various ingredients from which the particular paint color is derived.

Transparent and opaque glazes are both thinned with water or a glazing medium. The opaque glazes tend to be ones that are lighter in value, and have some white in the composition of the paint, which is what makes them partially opaque or milky-looking over top of your painting. Transparent glazes tend to be darker in value and are absent of white. 

A transparent glaze is a watered down, more transparent color used as a wash. Often these transparent washes tend to move back in space, relative to opaque glazes, because they are made of more transparent colors.