Marijuana and hemp both come from the genus Cannabacae. Marijuana plants are typically strains or hybrids of Cannabis Indica and Cannabis Sativa. Hemp is most often a strain of cannabis sativa or a hybrid of sativa and Cannabis Ruderalis.
The distinction between hemp and cannabis comes from the makeup of a particular cannabinoid called delta-9-tetrahydrocannabinol or THC.
Hemp is a cannabis plant with less than .3% THC. It shares many of the same cannabinoids that marijuana has but in different degrees of prominence. Hemp in its many strains have been grown to yield high levels of cannabinoids like CBD, CBG, and CBN, as well as other plant products such as hemp seed oil and fiber.
Marijuana strains have been grown to increase the yield and levels of THC. This is done to produce a product that causes the "high" associated with marijuana.