Varethane is a trade name for a brand of polyurethane varnish. Polyurethane varnish ("poly" for short) has urethane plastics in it in addition to the varnish resins. Regular varnish doesn't have these man made ingredients in it. However, Poly and regular varnish are compatable with each other since the base for either is varnish. Poly has better wear resistance than spar varnish due to the urethane additives.

UV inhibitors are also man made ingredients which can be added to either varnish or Poly to slow/prevent ultra violet damage to the underlying surface.

FWIW, Poly and spar varnish are interchangeable with each other for most applications where a varnish is used.