Hemp is the non-psychoactive variety of the Cannabis plant. There is a lot of confusion around what hemp is, due to the propaganda machine compelled onto the people for the last 75+ years. Industrial Hemp is classified and accepted, generally, as any part of the cannabis plant that contains less than 0.3% THC (Tetrahydrocannabinol) the psychoactive cannabinoid. Therefore, this could be the seed, stalks, leafs or flowers of a variety of cannabis that is very low in THC - which will not get a person high if consumed even in large amounts.
Physical Characteristics of Industrial Hemp:
Hemp is similar to jute or flax in its appearance but differs in that it grows to be roughly 12 - 16 feet tall. Hemp leafs are a rich chlorophyl color and the stems or stalk of the plant are a light brownish green color. Hemp is a plant that has prossibly the longest history of use more than any other plant. Humanity has used it for everything from clothing to ropes to food to shelter and so much more.
Industrial hemp, depending on what it's grown for, will display different characteristics. If grown for fiber products such as ropes, twine, textiles and other woven applications it grows very tall and about the stalks being about 1 inch apart from each other. If grown for seed the plant may be grown a bit further apart and have larger flowering tops where the seeds are produced.
Physical Characteristics of Medical or Psychoactive Cannabis:
Although, Cannabis is a term used for both the industrial and psychoactive applications of cannabis generally when the word cannabis is used by itself it is referring to the psychoactive variety, although that's not always the case.
When cannabis is grown for medicinal purposes or spiritual/psychoactive uses it is grown much further apart from each other. The stalks need a lot more room to branch out, resulting in larger flowers which is where the resinous cannabinoids are predominantly found.
Also, the aroma is much more pronounced, as the medicinal cannabis produces rich terpenes and other aromatic compounds. Although at one point the general idea was that hemp (or industrial cultivars of cannabis) can be easily distinguished from medical (or medical strains of cannabis) it is now confirmed that cannabis growers are able to grow "industrial hemp" varieties as they contain less than the accepted amount of 0.3% THC.