Different chemical composition:
Both hemp and marijuana belong to Cannabis Sativa family and look the same. The crucial difference between hemp and marijuana is the chemical composition. Hemp contains less than 0.3 % of THC whereas marijuana contains 5% - 35% of THC. THC is a psychoactive component in marijuana that gets people "high" after inhaling it.
The other way to distinguish hemp from marijuana is by looking at its physical appearance.
Hemp plants are taller, almost 20 ft in height. While marijuana comparatively looks shorter with bush-like an appearance.
Different methods of cultivation
Marijuana is grown in a controlled environment, like a greenhouse.
Industrial hemp is grown in a less controlled environment, like an open field.