Marijuana refers to the dried leaves, flowers, stems, and seeds from the hemp plant Cannabis sativa, which contains the psychoactive (mind-altering) chemical delta-9-tetrahydrocannabinol (THC), as well as other related compounds. This plant material can also be concentrated in a resin called hashish or a sticky black liquid called hash oil.
Marijuana is the most common illicit drug used in the United States. After a period of decline in the last decade, its use has been increasing among young people since 2007, corresponding to a diminishing perception of the drug’s risks that may be associated with increased public debate over the drug’s legal status. Although the federal government considers marijuana a Schedule I substance (having no medicinal uses and high risk for abuse), two states have legalized marijuana for adult recreational use, and 20 states have passed laws allowing its use as a treatment for certain medical conditions.
via DrugFacts: Marijuana | National Institute on Drug Abuse (NIDA).