All three come from the cannabis plant. The cannabis plant has been used for thousands of years. But cannabis usually refers to the THC-containing form.
There are three species of cannabis: Cannabis sativa, Cannabis indica, and Cannabis ruderalis. There is some dispute if these are really different species, but they are all commonly used names. Industrial hemp comes from Cannabis sativa. All three species can be used to produce CBD and THC.
Hemp generally refers to the industrial uses of the plant, mainly referring to the use of hemp fibers. Hemp fibers can be used to make plastic substitutes, building materials, clothing, ropes and many other things. Industrial hemp has low levels of THC, the psychoactive substance in medical and recreational cannabis. Hemp seeds and other hemp plant parts can also be used to make oil which can be used for cooking and other purposes.
CBD is cannabidiol and is one of the substances in the cannabis plant. CBD has health benefits such as being an anti-inflammatory. Usually CBD comes from cannabis plant varieties with low THC grown specifically for the CBD content. CBD can be used in many different forms such as in creams, pills or in liquid form. Different countries have different laws and regulations about how much THC is allowed to be in a CBD preparation. Usually, it is well below 1% THC.
Medical and recreational cannabis (also known as marijuana) typically has higher THC and the main part of the plant used is the flower, also known as the bud. Since THC is psychoactive, it is considered to be a narcotic and is tightly controlled by government laws and regulations. Most THC-containing preparations also contain CBD, other cannabinoids and terpenes.