Polyurethane is like varnish in the way that it is a transparent, protective substance that is usually used to coat wood. The polymers in the polyurethane are what cause it to have exceptional durability.  There are two different types of polyurethane – oil-based and water-based which both have their pros and cons. Polyurethane that is oil-based is more durable but can take a long time to dry which often takes more than a day to finish two coats.  You can expect to spend several weeks waiting for oil-based polyurethane to finish its two-phase hardening process which is also known as curing. Water-based polyurethane is not as durable, but it dries in just a few hours and cures in around two weeks. It also smells a lot less harsh than oil-based polyurethane and contains little to no VOCs (volatile organic compound emissions) but you should still wear a ventilator mask when working with it. Both varnish and polyurethane are applied in the same way and contain a lot of the same ingredients.  They also look the same once they have dried as they are both transparent but in terms of levels of protection polyurethane is a bit more resistant to abrasions, but varnish does better at preventing UV light damage and harsh weather. Both varnish and polyurethane are waterproof and do well at preventing rot, mildew, fungus, and pests. If you live in an environment that is non-climate controlled, then you should go with varnish instead of polyurethane as varnish allows the wood to breathe and flex.  Polyurethane is still a good choice for indoor furniture though and varnish is best suited for exterior use as the odor can be very unappealing.