Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Piperales
Family: Piperaceae
Genus: Piper
Species: P. methysticum
Kava refers to the species P. methysticum and at least 50 other species of the family Piperaceae. But if you are talking about kava the drink/drug/'herbal remedy', it's almost surely P. methysticum, the most popular of the psychoactive kava species.
Kava root has been used by various groups of Pacific islanders for over three thousand years as a relaxing and stress relieving drink. Kava is traditionally used as a drink for social gatherings, and in some cases has religious uses.
Kava drink (sometimes called grog, although it is not alcoholic) is prepared with cold water and kava powder, pounded from kava root. It relieves stress, relaxes the body and imparts a feeling of well being. Used before bedtime it promotes restful sleep (although some report having very vivid dreams after drinking kava). It may also cause localized numbing in the lips and tongue.
Kava is just now becoming popular in America. It has a large following in the surfer subculture, and can often be found in alternative medicine contexts. It is legal in America, but the FDA is has issued a warning about possible liver toxicity. It is banned in some European countries, and kava derivatives are banned in the UK.
As I recall, kava tastes like water that has had a bit of old wood soaked in it. Nothing special. It is sometimes combined with coffee to produce "kavajava". I have never tried this, but it is said to be better than either kava or coffee on their own.