Galangal (Thai: ข่า, Malay lengkuas, Mandarin 高良薑(Traditional)/高良姜(Simplified), Cantonese lam keong, also known as blue ginger), is a rhizome with culinary and medicinal uses, best known in the west today for its appearance in Thai cuisine (e.g., tom kha gai) but also common in recipes from medieval Europe. It resembles ginger in appearance and taste with an extra citrus aroma and a slight hint of soapiness. It is available as a powder from vendors of Oriental spices and also available in whole, cut or powdered from vendors of herbs. Also known as galingale, kencur, or laos (its Indonesian name). Coincidentally, it is one of the most prominent herbs in Lao cuisine.
The word galangal, or its variant galanga is used as a common name for all members of the genus Alpinia, but in common usage can refer to four plants, all in the Zingiberaceae:
1) Alpinia galanga or greater galangal
2) Alpinia officinarum or lesser galangal
3) Kaempferia galanga, also called lesser galangal or sand ginger
4) Boesenbergia pandurata, also called Chinese ginger or fingerroot
A galanga is also known as Chewing John, Little John Chew and galanga root. Under these names, it is used in folk medicine and in voodoo charms (see John the Conqueror for related lore).
Extracted from : http://en.wikipedia.org/wiki/Galanga