Cannabis sativa and Cannabis indica contain a wide range of natural substances (THC, CBD, CBN) that have both psychoactive and physical effects. For some people, these effects are not only enjoyable but useful, beneficial and a source of relief.
When preparations of cannabis have been prescribed by a licensed doctor and provided by a pharmacy, it is often referred to as "medicinal marihuana". Although the strength of these products may be comparable to what's available on the street (or one's backyard), they are guaranteed pure and free from mould.
In the United States (especially in California) many people have a prescription for medicinal marihuana. In the Netherlands, it's possible to get prescribed cannabis, but since various strains are readily available in "coffeeshops", not many people make use of this system.
More information can be found here: Scientific information on vaporizers and cannabis