Galangal, galanga, or galingale - also sometimes called laos - is an aromatic root or rhizome used in Thai cooking. It looks like very fresh young ginger and has smooth pale gold-brown skin and pale creamy white flesh. Galangal has a very distinctive cedary, medicinal smell and a hot, gingery-peppery flavour; though recipes sometimes suggest that you substite ginger for galangal, I wouldn't recommend it; they don't taste the same at all.
Actually, there are two types of galangal, greater and lesser. Unless you live in Southeast Asia, chances are the greater form is all you'll see. The lesser form has orange flesh and is hotter, but I've never seen it outside of Thailand. You're only likely to be able to buy galangal in your friendly local Chinatown, but it can be hard to tell what's what; the best way I know to tell the difference between galangal and young ginger in the absence of cultural interpreters and labelling I can read is the price: ginger is sold in bulk at very cheap prices, while galangal is sold in small knobs at substantially higher prices. You may see dried galangal for sale, but pass it by; its flavour is but a shadow of the fresh version. Instead, freeze leftover fresh galangal; it retains its flavour well when frozen.
In Thai cooking, galangal is an essential ingredient in soups such as tom yam and tom kha kai; ground, it is also an important component in curry paste.