Galangal, ginger and turmeric are all rhizomes commonly used in Asian cooking.

All three are also used in herbal medicine. Ginger has long been used a cure of colds and coughs. Over the past decade, turmeric has gained popularity for its supposed anti-inflammatory properties. But this is a food blog so I’ll go ahead and skip the alleged health benefits of all three.
Differences in appearance
There are various types of galangal but, unless you’re a botanist, only two are worth getting acquainted with — Lesser galangal and Greater galangal. What you see in the photo above is Greater galangal which is a popular ingredient in Thai cooking.
Greater galangal and ginger are, on the average, of the same size. But the skin of galangal is smoother and with more pronounced “rings”.
Turmeric, on the other hand, is smaller and the skin is darker and rougher.
Once you peel off the skin, the interior of the galangal can be anywhere from white to pale pink. The inside of ginger is yellow. Turmeric’s is deep yellow to yellow-orange.
The texture of each varies too. While turmeric and ginger are easy to grate, galangal is too fibrous — almost woody, in fact.
Differences in aroma and flavor
Galangal is citrusy and somewhat piney. Unlike ginger, there is no pronounced heat.
Ginger is the most aromatic of the three (please don’t use the word pungent to describe the aroma). It also has the highest amount of heat.

Turmeric is the least aromatic of the three. It is mildy earthy and slightly bitter. In cooking, turmeric is often used to add color to dishes.
How to buy galangal, ginger and turmeric
All three are available fresh, dried or powdered. If you’re lucky, you may also buy them in grated form in jars.
When buying fresh galangal, ginger and turmeric, always choose ones with the smoothest skin and with no dry and withered-looking spots.
Can one be substituted for the other?
Bon Apetit and Food Republic are both grossly mistaken in saying that ginger is a worthy substitute for galangal. A DESPERATE substitute, yes, but not worthy. I know because until I was able to get galangal (either crushed in jars or dried), I used to substitute ginger. Then, I learned and never made the mistake of substituting again.

As mentioned before, galangal, ginger and turmeric all have different flavors and aromas. Unless you want to change the traditional flavors of a dish, you need to use the correct rhizome intended for it.