Diazepam is a benzodiazepine which has a relatively long half-life. It is mostly used to treat anxiety, certain forms of epilepsy, muscle spasms (where the muscle moves on its own, causing pain), and sleeping problems. Like with other benzodiazepines, there is a risk of addiction if used for long periods of this. For this reason the drug is used to treat acute cases (2-4 weeks maximum) to avoid dependence. According to the World Health Organization diazepam is an essential drug.
Diazepam is marketed under a number of names; its original trade name was Valium. It has been the most prescribed medicine in the United States many times. It is sometimes used recreationally to cause a calming effect.