Hemp and marijuana are the same species of plant — Cannabis sativa. Like other plants, cannabis has many different forms (called phenotypes) depending on its genetic lineage and growing environments. Even though two plants may be the same species, they can be exceptionally different from each other. The main difference between hemp and marijuana is the amount of THC they produce. THC is the psychoactive ingredient in cannabis that makes users feel high. Officially, hemp is any plant that contains less than 0.3% THC by dried weight. Marijuana is any cannabis plant containing more than 0.3% THC. Hemp is completely legal in the United States and much of the world, while marijuana is generally considered illegal due to its psychoactive effects.