Cannabis, cannabinoids, and marijuana all describe the drug.
Cannabis, Cannabis sativa, and Cannabis indica describe the plant.
THC is short for tetrahydrocannabinol, also known as ∆-9-tetrahydrocannabinol (pronounced delta-nine-tetrahydrocannabinol). It is the psychoactive component of cannabis, responsible for generating a high.
Cannabidiol is a non-psychoactive component of cannabis, studied for its medical applications.