CBD, short for cannabidiol, is a natural cannabinoid contained in the plants of the cannabis family. Both hemp and marijuana belong to this same cannabis plant family, but they are two distinct plants (technically, not taxonomically) with different CBD content.
Marijuana contains both CBD and quite a bit of THC. THC is an illegal substance in most states and countries as it causes an addictive high and has a potent psychoactive effect.
Industrial hemp, on the other hand, contains very little THC and a lot more CBD.
Patients who want the combined effects of THC and CBD to medicate their symptoms prefer high-CBD marijuana strains, while patients who want to medicate only with the non-psychoactive CBD prefer high-CBD hemp oil