To clear up any confusion, of which there is much, Hemp and Cannabis are the same plant species - Cannabis sativa L.
The difference in definition is actually quite simple - all varieties that contain negligible amounts of the psychoactive chemical THC < 0.3% are known as hemp.
Any variety of cannabis above this level of THC can be called any one of a plethora of names from marijuana, pot, dope, weed, grass, ganja etc. and according to Wikipedia up to 1200 more slang terms. Check them all out here.
CBD is produced in both low and high THC varieties of the plant but crops that are grown for industrial purposes and for fibre, bio-fuel, building materials, textiles and seeds have low THC content but still contain significant levels of CBD.
We made a decision to use the name cannabis where we can in our stories, because it’s the plant’s real name, but we may use hemp in some contexts because it’s easier to define as the raw material, and because even the word cannabis still makes some people nervous.