Galangal is a bit bigger than ginger, with shinier, whiter skin, while also being much firmer. Galangal is hard, woody, with a softish centre – a sharp knife is needed, or a good spice grinder.
Galangal tastes sharper, almost citrus-like, adding good flavours to dishes.
Ginger is NOT a substitute for galangal. Sometimes you can find galangal powder which isn’t quite as good as fresh galangal root. It is also possible to find galangal in Asian grocer freezers.