The major difference between hemp and marijuana is that hemp has low levels of THC.
While marijuana is bred with the sole purpose of maximizing its THC concentration, hemp always has trace amounts of THC and naturally occurring high amounts of CBD.
The legal difference between Hemp and Marijuana is if a plant has less than .3% THC it is considered Hemp and legal in all 50 states. If a plant has over .3% THC it is considered Marijuana and illegal in most states.
Hemp has tall, fibrous stalks that are very strong and have very few flowering buds. Marijuana strains are shorter, bushy, and have high amounts of THC.