Resins are amorphous solid products of a complex chemical nature. They are a heterogenous group with certain well-identified physical, chemical, and solubility characteristics. Resins are formed in schizogenous or schizolysigenous ducts or cavities by cells, which secrete a fluid composed of substances constituting gums, essential oils, and resins. Resin is held in solution by volatile terpenes, which are secreted in association with it. Thus resins are often associated with essential oil (turpentine, copaiba), with essential oil and gums (myrrh, asafetida), or with balsamic acids—benzoic and cinnamic acids (Benzoin, Storax) or are present as glycosides with sugars (Jalap, Podophyllum). Resins from which the associated essential oil is removed are called rosins.

They may be preformed in plants as normal physiological products and their yield is enhanced when an injury is made to the plant tissue. Many resins such as benzoin are formed only in response to injury. Resins may be present in idioblast cells (ginger oleoresin), multicellular internal glands (Clove), external glandular trichomes (Cannabis), tubular ducts with secreting epithelium (Fennel, Pine wood) throughout the tissue (Guaiacum heart wood) or they may be formed from plant juices by the agency of insects (Shellac in glands on the lac insect). Resins may be oxidation products of terpenes.

Plant resins have a long history of use and because of their physicochemical properties and associated uses, resins were used as medicine, in paper sizing, and in the production of varnishes, adhesives, food-glazing agents, incense chemicals, and as source of raw materials for organic synthesis. While most of these uses are replaced by synthetic substitutes for resins, some resins still hold a valued role in pharmacy for their medicinal and unique physicochemical properties. Medicinally used resins have been associated with anti septic, carminative, astringent, stimulant, diuretic and laxative properties.

Resin production is widespread in nature, but only a few families are of commercial importance. These include the Anacardiaceae, Burseraceae, Dipterocarpaceae, Guttiferae, Hammamelidaceae, Leguminosae, Liliaceae, Pinaceae, Styracaceae, and Umbelliferae.