Same botanical family but so different in taste and use in cooking various Asian dishes!
There are around 400 members of the ginger family grown wild in the tropical Asia but these 2 particular variety is the one universally known as Ginger & Galangal.
A young ginger has a very thin skin, is pale yellow and has pinkish shoots with green stalk ends, while old ginger is beige-brown with a thicker skin. Young ginger is more tender and juicy than the mature rhizome, so it is preferred for grating or pounding to extract the juice, a popular marinate with Chinese chefs. It can be eaten raw, and is also pickled. Mature ginger although sometimes served raw in very fine shreds, is more commonly cooked as the flavour is more emphatic than that of young ginger. Ginger is widely used for medicinal purposes throughout Asia, particularly to improve digestion and to counteract nausea and vomiting.
The galangal has a pungency and tang quite unlike that of the common ginger. The young shoots of the rhizome are pale pink and are more flavourful and tender than the older beige- coloured rhizomes. Galangal is too spicy to be eaten raw, and is used in slices, chunks or pounded to a paste for various curries and side dishes. When pounding or blending galangal to a paste, first shop it into small pieces as it is often obstinately tough. Perhaps this is why Thai cooks often just bruise a large chunk with the flat side of a cleaver and add it whole to the cooking pot.
Article Source: Tropical Spice Garden Cooking School