Varnish is old school poly is new. Varnish has eithe4 linseed or tung oil in it as a base.
Therefore, varnish is a material which consists of resin and solvent. You put it on wood, the solvent evaporates, and the resin is left. Polyurethane (the Minwax at Lowe's) is a type of varnish, although...