Terpenes are plant chemicals that are extracted through distillation (essential oils), solvent extraction, and cold pressing. Terpenoids is term for terpenes with a functional group. In essential oils, there are monoterpenes and sesquiterpenes, which have various actions on skin - antinflammatory, antioxidant, and antibacterial. Triterpenes are too heavy to make it to essential oil but they can be extracted in lipids. This is why we infuse calendula in oil and why it is so good at repairing dry skin. Squalene, found in olive and argan, is another good example of a triterpene.