Varnishes are composed of three ingredients: resin, plasticizer and solvent.
Resin is what gives varnish its hardness. It can be natural (dammar, copal, mastic or rosin,) made from an animal base (shellac,) or synthetic. Natural resins are produced by plants to help heal their injuries. Varnishes made with natural resin can yellow over time or become brittle. Synthetic resins have the advantage of never yellowing, and of being completely impervious to humidity. Varnishes made with hard natural resins, such as dammar resin, create a very hard finish, but they may have a tendency to yellow. Modern synthetics were developed to avoid the risk of discoloration, and as a result are a safer choice.
Plasticizers add flexibility to hard resins. Linseed oil and Venetian turpentine often serve this purpose.
Solvent helps dissolve the resin, and evaporates as the varnish dries. The most common solvents are turpentine, mineral oil and methylated alcohol (ethanol.) When you use a varnish, make sure to check what solvent it’s made with so you can choose the appropriate solvent for cleaning and diluting if necessary.