Both stainless and galvanized steels prevent corrosion by forming a "sacrificial" layer of protective oxides. Stainless forms an almost invisible layer of chromium oxide and galvanized forms a (comparatively) thick layer of zinc oxide, according the Metallurgical Consultants website.
Eventually, the galvanized steel's zinc oxide coating will wear away, leaving it vulnerable to corrosives and weather. Stainless steel contains chromium all the way through, so its protective layer is continually replenished.
Stainless has a shiny finish with a deep luster, and can also be finished with different grits to give different appearances. Galvanized material is always gray and lacks any shine.