Both THC (in marijuana) and CBD (in hemp) belong to a class of plant chemical compounds called cannabinoids that bind to receptors in the endocannabinoid system. The endocannabinoid system modulates endorphins (chemicals that modulate the pain response), the balance of neurotransmitters in the brain, immune system functions, emotional centers, and metabolism.
THC binds to endocannabinoid receptors very tightly, causing an exaggerated or euphoric response — in other words, you get high. In the process, it also suppresses endocannabinoid receptors and levels of natural endocannabinoids in the body.
CBD, on the other hand, weakly binds to endocannabinoid receptors in the brain and body, gently stimulating and blocking them at the same time. This has positive results without an exaggerated euphoric response, so you don’t get high or intoxicated in any way. In addition, regular use of CBD seems to increase endocannabinoid receptors and natural endocannabinoids.