Agricultural hemp is significantly different from marijuana, even the way it looks. Agricultural hemp has tall stocks and very thin leaves, with little to no flowers. It is defined as a cannabis plant containing less than 0.3% THC, hemp is grown in numerous industries from packaging to foods. Another major difference is that hemp does not get you "high".
Marijuana plants on the other hand tend to be very short and are grown for their flowers. The THC content is significantly higher and these plants tend to be used recreationally in states that allow it (CA,OR,WA,NV,MA,AK,CO & ME). It is also used medically if recommended by a doctor. Because of the restrictions around Marijuana, growing, selling and buying it is illegal in many states.