Cannabis and industrial hemp are the same thing. Strains of cannabis with THC concentrations .3 percent and lower are legal to grow as agricultural hemp.
Depending on the strain, CBD can be extracted from cannabis. Some strains bear more CBD than other strains.
Anecdotal evidence exists that suggests that without THC being present in some quantity, CBD is not properly activated. There are many adherents to this belief. Keep in mind that there are many who believe in magical deities who are all places at once.
CBD extracted from cannabis is the same thing no matter what level of THC is present in a given strain. THC may be a force multiplier when combined with CBD.
There is limited data available at this time which can scientifically explain exactly how CBD functions in the human body.