My interpretation is that a glaze is a transparent or semi-transparent pigment applied over an underlying painted surface to acheive a purpose (shadow, highlight, lowlight, corrosion, etc.). A glaze may be applied to a limited surface of a model to acheive that purpose. A wash may be broadly considered to be a glaze, but my understanding is that a glaze is applied using a more controlled manner than a wash which is flooded on. A glaze may be made using a varnish (clearcoat) and a pigment.
A varnish is a clear (gloss, satin, matte/flat) coating applied as a final top coat or as a protective coat between various stages of completing and finishing a model. Varnishes may be either organic (lacquer, polyurethane, solvent based) or acrylic (water, alchohol based). Varnishes are generally applied to the entire model.