There have been over 100 terpenes found in cannabis, and the extent to which plant terpenes in general have been studied means a fuller knowledge of these compounds than cannabinoids (which have obviously not been studied so deeply). A patient armed with knowledge of these stinky bastards will be better equipped to treat their specific ailments. Finding the right strain for you can be a hit-and-miss journey otherwise.
Terpenes are a class of organic compounds found in many plants and even some insects. They often have a strong odor and are thought to be produced to attract pollinators and to protect against predation by attracting predators that feed on herbivores.