Cannabis refers to a whole family of plants in the Cannabaceae family, which includes  three primary species: Cannabis sativa, Cannabis indica, and Cannabis ruderalis.
Hemp is a term used to classify varieties of Cannabis that contain 0.3% or less THC content (by dry weight).
Marijuana is a term used to classify varieties of Cannabis that contain more than 0.3% THC (by dry weight) and can induce psychotropic or euphoric effects on the user.