While hemp and marijuana are very different, they share a lot in common.
Both come from a plant species called Cannabis sativa. However, like many plants, the difference in “cultivar” or “strain” means a lot. Strain type determines everything from plant height to cannabinoid ratio to industrial applications. Over decades, these cultivars have been bred to possess specific, desirable traits. In recent history, breeders have been after the highest possible THC content. However, centuries ago, cannabis sativa was sought after for its fibrous content and industrial applications (like paper production). The cultivar that grew in popularity then is what we know today as industrial hemp.
While marijuana and hemp come from a similar plant, they have some key differences. Most notably, marijuana is high in THC while Hemp has virtually no THC. This gives more people in more places access to the powerful cannabinoids in hemp.