According to the number of isoprene units (c5 units), common terpenoids can be divided into monoterpenes, sesquiterpenes, diterpenes, sesquiterpenes, triterpenoids, tetraterpenes and polyterpenes. Each category is further classified according to whether the basic carbon chain is looped and the number of loops.

monoterpenes: monoterpenes can be regarded as compounds polymerized from two isoprene units and their derivatives, which are components of volatile oil. Most of them have strong aroma and physiological activity. For example, chain monoterpene geraniol has antibacterial effect; Monocyclic monoterpene peppermint ketone has antiasthmatic, antitussive and antibacterial effects; Bicyclic monoterpene borneol (borneol) has the effects of sweating, excitement, antispasmodic and anthelmintic.

sesquiterpenes: sesquiterpenes are compounds and their derivatives polymerized from three isoprene units, which can exist in volatile oil and have aroma and biological activity. Monocyclic sesquiterpene artemisinin has anti-malarial effect.

diterpenoids: diterpenoids are compounds and their derivatives polymerized from four isoprene units. The molecular weight increases, and most of them are non-volatile. Ginkgo biloba lactone, a bicyclic diterpene, is an effective drug for treating cardiovascular diseases, and andrographolide has antibacterial and anti-inflammatory effects. Tricyclic diterpenoid triptolide has anti-cancer, anti-inflammatory and anti-fertility effects. Tetracyclic diterpenoid stevioside can be used as a sweetener for patients who are forbidden from sugar, and its sweetness is 3 times that of sucrose; Aconitine, a pentacyclic diterpene, has analgesic, local anesthetic, cooling and repercussive activities.

Triterpenoids: Triterpenoids are compounds and their derivatives polymerized from six isoprene units. When it exists in free state, it is called triterpenoid compound or triterpenoid aglycone, and when it is combined with sugar, it is called triterpenoid saponin