Mediums and Varnishes can be used for all types of things. Effects mediums, like Acrylic Gel and Paste Mediums, thicken and change the paint consistency. Acrylic Varnish is usaully a polymer glaze. Oil Mediums do similar things to the paint just bond with oil.
Additives change the paint properties, making it dry slower or faster, or improving its flow. Oils like Linseed, Alkyd, and stand do similar things to oils.
Primers and Grounds, Waxes, and Texture Medium, and Thinnner, we have it all.