Hemp and marijuana are both part of the same family of the cannabis plant. There are three main varieties of cannabis. Sativa, Indica and Ruderalis.
Cannabis sativa and cannabis indica are most widely known and widely used for the cultivation on hemp, also called industrial hemp. The same varieties are also used for the cultivation of marijuana sometimes called, weed, ganja, reefer, etc.
Although both plants look and smell the same, the difference between hemp and marijuana is down to the amount of the compound THC found in each plant.
Hemp plants contain no more than 0.3 percent (by dry weight) of THC (tetrahydrocannabinol), the psychoactive substance found in the cannabis plant.
Simply put:
Cannabis plants with 0.3 percent or less of THC are hemp.
Cannabis plants with more than 0.3 percent THC are marijuana.
Cannabis: the plant with many names
The names given to the various strains of cannabis plants differ from region to region and country to country. The main thing to remember is that one strain is used for industrial purposes and the other for medical and/or recreational purposes.
Disclaimer: It is important to check the laws regarding these plants in your region as laws may differ from country to country and even region to region within the same country.