Varnish is a clear, transparent hard protective finish or film. It is neither paint nor stain. In its native state, it has little or no colour, but can be pigmented as desired, and is sold in various shades.
Varnishes provide protective coatings for wooden surfaces, paintings, and various decorative objects. Varnish protects and enhances the appearance of wooden floors, interior wood panelling and trim, and furniture.
A sealer is a special type of primer designed to be used with, or in place of, a primer. A sealer prevents the topcoat from soaking into the substrate, and to a degree, can create a water-resistant layer between the substrate and topcoat.