Tanning is the process by which an organic material (animal skin) is transformed from organic to inorganic, meaning it is no longer susceptible to warm water or bacteria. Nowadays, most tanneries use chrome, because it produces softer leather with the best physicals.
Vegetable tanning used to be very popular. It is more environmentally friendly but the physicals aren’t as good. For example, the leather will change color quickly when exposed to sunlight. However, vegetable tanned leather does age and burnish well, so tanneries will often decide between chrome and vegetable tanning depending on the finish they intend to use. Lastly, wet white tanning involves using pure organic chemicals for environmental.