Resin is thought to plant tissue or secretions of normal metabolites, often, and essential oil secretion coexist in plant cells, resin ducts , or conduit, especially perennial woody parts of the catheter in the heartwood. Mixture by a variety of ingredients, usually amorphous solid, slightly shiny surface, hard and brittle, a small number of semi-solid. Insoluble in water, not water-swelling, soluble in alcohol, ether, chloroform and so most of the organic solvents . Heating to soften, and finally melting, burning smoke from time to time, and have a special aroma or odor.  Natural resins and synthetic resins into two categories.