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 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 color is derived.

Opaque and transparent 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 absent of white. 

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