Related, but different.
The hemp plant is used to describe a variety of cannabis that contains the THC component (about 0.3% or less). The hemp is also described as the non-intoxicating aspect of cannabis that can be used for various industrial purposes when processed. The hemp plant has several uses that span from health, industrial manufacturing and known to contain the highest amount of fiber.
Marijuana is used to describe a variety of cannabis that contains more than 0.3% of cannabis concentration and often taken as THC pills. Besides, it can also cause some psychoactive effects on the user. However, to date, cannabis is still the primary term used.