The primary difference between THC and CBD is the fact that THC is psychoactive and CBD isn't. Psychoactive drugs are when taking or ingesting these compounds will create psychedelic and psychoactive effects on the brain. These effects that you experience are the reason why it is an illegal substance currently.

They have more in common than they do have separate, however. Both of these compounds are cannabinoids. This means that the same parts of your body that can be positively affected by CBD will also be effective by THC.

Since most of these areas are parts of the body as well as parts of the nervous systems, this results in the 'body high' feeling that many users of THC report.

What's the difference between using THC and using CBD?
The ECS or Endocannabinoid system is the network of Cannabinoid receptors around the body that absorb CBD and THC. However, the network themselves is a little more complex. While the receptors are all endocannabinoids, they are broken into two systems Cannabinoid Receptor 1 and 2 (known as CB1 and CB2.)

While the receptor is one singular unit, there are two sites with which cannabinoids can bind: The orthosteric and allosteric sites. THC binds to the orthosteric while CBD binds to the allosteric sites respectively.

While many states where recreational marijuana is legal to have had much success in altering strains to balance the effects on both receptors, strictly CBD trials have had a harder time pushing through with their studies.

Despite the setbacks for some fields in CBD medicine, there have been numerous studies on the effects that CBD has on the body and the ECS. 

Some of these studies have proven to produce positive effects strong enough to warrant practical usage of CBD to treat ailments such as Cancer, Diabetes, and Lupus. Read more about what CBD is used to treat here.