The scientific name of Marijuana is Cannabis Sativa or Indica. Marijuana users consume its leaves, seeds, stems, and/or roots for the purpose of feeling intoxicated ("high"). This plant contains hundreds of compounds, but the one that has major intoxicating effects is called tetrahydrocannibinol (THC).
It is one of many illegal drugs in most jurisdictions But legal in a few states of the United States.
Specifically, laws in most states deem it illegal to engage in possession of marijuana, either for the purpose of distributing it to others or for your own use.