First of all Hemp is Cannabis. Marijuana and Hemp are both of the Cannabis Sativa L genus.
The Federal Government defines Industrial Hemp as : "the plant Cannabis sativa L. and any part of such plant, whether growing or not, with a delta-9 tetrahydrocannabinol concentration of not more than 0.3 percent on a dry weight basis."
With that said there are many different types of industrial hemp but we can break them down into two major categories.
You have feral hemp, which is hemp as most of us know, tall lanky seeded varieties that have been bred for fiber and seed production.
You now also have Medicinal Hemp. Medicinal hemp is essentially for all intents and purposes, strains that have been bred to have low THC and high CBD or other cannabinoids that have high medical properties.
Feral hemp is a poor choice for CBD extraction due to it having a very low percentage of CBD .
You must use more plant material and this also increases the potential for higher levels of contaminants in your extract. Hemp absorbs contaminants from the earth and in other parts of the world is used to clean contaminants from the soil.
Medicinal Hemp requires less material as it is usually around 15-18% CBD by weight. Imported products that claim to be from stalk and seed are typically from Feral Hemp and tend to have higher levels of contaminants.