Although they are both of the complex cannabis genus, the differences between marijuana and hemp are many, and start with the seeds: Marijuana strains have been bred to have elevated THC and low CBD hemp oil, whereas hemp is bred to be just the opposite.
The ways in which marijuana and hemp are cultivated vary greatly as well. Cannabis is a dioecious herb; marijuana growers will remove the male plants from the “grow” area, carefully cultivating only female plants for their flowers. These female plants are generally grown to be “bushy” to promote an abundance of flowers, which are what are harvested for marijuana.
Hemp agriculture is quite different. Male and female plants are grown very close together, and branching is discouraged, resulting in tall, thin plants more suitable for fiber, and allowing for easier wind pollination, which is what is needed to ultimately produce hemp seeds.