Basically, cannabis is an umbrella term that includes both hemp and marijuana plants. Cannabis sativa is the scientific name of the cannabis plant (Cannabis = genus; sativa = species), and hemp and marijuana are just two different strains of it. (One could correctly refer to a “marijuana” or “hemp” plant as a “cannabis” plant, for example, but you would not refer to a “marijuana” plant as a “hemp” plant, or vice-versa).
Hemp seed oil is extracted from the seeds of the hemp plant typically using a cold-press process. This type of oil can actually be extracted from all plants in the cannabis genus, but only industrial hemp is used in production. Industrial hemp is used primarily because they have been specifically bred for seed and stalk. Hemp seed oil can be used as a substitute for olive oil and other cooking oils in various recipes and in salad dressings.