Marijuana comes from the cannais sativa plant, and is the dried and shredded leaves, stems seeds and flowers. The high you get from marijuana comes from a chemical called Tetrahydrocannabinol, also known as THC. Some strains contain more or less THC — making them more or less potent.
Most of THC's effects happen in the brain, where the chemical interacts with receptors on brain cells called cannibinoid receptors. Our bodies actually make chemicals very similar to THC, which are used in normal brain function and development. THC co-opts these natural pathways to produce most of its effects.
The drug is also touted for its uses for multiple diseases.