Short Story: Cannabinoids are chemical compounds that interact with the body. THC and CBD are 2 of over 100 cannabinoids found in marijuana. Cannabinoids are also produced by the body.
Long Story: Endocannabinoids, phytocannabinoids, and synthetic cannabinoids all interact with the body. We actually produce our own cannabinoids (endocannabinoids) that interact with our Endocannabinoid System. In fact all complex creatures have this system and produce cannabinoids (yes your dog!). Many plants produce cannabinoids, including the black pepper, echinacea, chocolate, and I would speculate more will be identified in the next decades as research continues in this area. Synthetic cannabinoids, produced in laboratories, use the same chemical structures to copy the chemical structures of natural cannabinoids.
(Phyto)cannabinoids have complex scientific names, which is why most have been reduced to a simpler 3 letter shorthand. THC is short for delta 9-tetrahydrocannabinol as CBD is short for cannabidiol, etc. Many people are looking for either high THC or high CBD. In fact both were present in strains presented by nature before the plant was hybridized for recreational use. Although choosing between these 2 components can be a starting point for medicating, perhaps a well-rounded strain with a variety of cannabinoids is the best strategy. Few labs test for more cannabinoids, but look for CBC, CBN, CBG, and THCVas all of these work together and by focusing on one, the plant may lose the benefit of the others.
It is very difficult to find a comprehensive list of all cannabinoids. Here are a few links to the most common ones.