Marijuana is a species of cannabis (the same plant used to make hemp rope) that has a high concentration of oils containing THC. THC is an intoxicant that has a host of effects. THC has analgesic effects, dulling pain. Anecdotal evidence suggests that it can reduce nausea. Some do use it recreationally for the high, and for the hallucinogenic effects it can occassionally have.
However, marijuana and THC do have consequences that are not to be taken lightly. THC has been shown to impair perception and judgement, similar to alcohol. Marijuana is also psycho-somatically addictive. Unlike Nicotine, which is psycho-tropically addictive, THC doesn't affect dopamine levels so coming down off the high doesn't trigger withdrawal symptoms; but the user may crave the sensation of the high leading to a desire to use again. THC is fat soluble, and so can remain in the body for years after last use, this can cause hallucinations and other high-like symptoms later on in life. THC is also suspected to be the cause of a number of birth defects.
Federally, Marijuana is illegal, and recreational use is illegal in 48 states. (Colorado and Washington are the exceptions). Some states allow medicinal use, which is heavily regulated and still federally illegal. Other states still hold fast to the federal rules and forbid possession and distribution.
It is a stimulant used for relaxation.