Cannabis sativa is the prototypic source for cannabinoids. THC was the first chemical compound found to affect the CB2 receptors in mammals. Over 120 different cannabinoids have been described as occurring in Cannabis, although many cultivars only contain a small number of these.
The term cannabinoid in this context refers to chemicals with a structure derrived from Cannabiberol (CBG), which is the precursor of all cannabinoids in Cannabis. Not all cannabinoids interact with the cannabinoid receptors. The term cannabinoid is used to convey a different meaning - it is not based on chemical structure, but on the activity of the molecule in an organism. Spice and K2 are examples of synthetic cannabinoids that look nothing like CBG or THC, but which interact with CB receptors in the body. Other synthetic cannabinoids (like HU-308) look very much CBG and THC from a structural standpoint.
These cannabinoids are known to interact with the CB2 receptor: