Leather is dried animal hide. The tanning process -- the set of chemical treatments that convert raw hide into leather -- stabilizes the collagen fiber structure so that it does not rot, remains flexible, and develops the surface characteristics that distinguish different types of leather from each other. The quality of the leather is determined primarily by which part of the hide is used and how it is tanned.
The hierarchy of leather quality starts at the surface. Full-grain leather uses the outermost layer of the hide with the natural surface intact. The grain surface is the densest part of the hide -- the fibers there are tightly interlocked, which gives full-grain leather its durability and the ability to develop a patina. Top-grain leather is similar but the surface has been lightly sanded or buffed to remove imperfections, which removes the densest fiber layer. Corrected-grain leather is sanded more aggressively and coated with a polymer to simulate a uniform grain -- it will not develop a patina and it peels. Bonded leather is leather scraps and fiber mixed with polyurethane -- it is not leather in any meaningful sense.
Vegetable tanning is the oldest leather production method and produces the leather that ages most dramatically. Bark tannins from oak, chestnut, and mimosa react with the collagen fibers over a period of weeks to months, producing a stiff, dense leather that is light tan when new and darkens to a rich brown over years of use and exposure to light and oil. Hermann Oak in St. Louis and Horween in Chicago are the two major American vegetable tanneries still in operation.
Chrome tanning is faster -- hours rather than months -- and produces softer, more uniform leather at lower cost. Chrome-tanned leather is what most shoes, bags, and upholstery are made of. It does not develop the same dramatic patina as vegetable-tanned leather, but it is softer and more consistent. Most of the leather you encounter in daily life is chrome-tanned.
Caring for leather is simple: keep it clean, keep it conditioned, and keep it away from prolonged heat and moisture. Clean with saddle soap applied on a damp cloth. Condition with neatsfoot oil, mink oil, or a beeswax-based conditioner after cleaning. The goal of conditioning is to replace the natural oils that evaporate out of leather over time -- without conditioning, leather becomes stiff and eventually cracks along stress lines.
Beeswax-based products (Sno-Seal, Obenauf's) add a waterproofing layer on top of the conditioned leather. They do not replace conditioning -- they seal it in. The sequence is: clean, condition, then waterproof if needed. Applying waterproofing to dry, unconditioned leather traps the dryness inside. Apply conditioner to warm leather: a few minutes in a warm room (not near a heat source) opens the pores slightly and allows the oil to penetrate more deeply.
Shell cordovan -- the dense leather from the rump of a horse, tanned by Horween in Chicago -- requires different care. Its fiber structure is so dense that most conditioning oils sit on the surface rather than penetrating. Use Renovateur or a cordovan-specific cream, applied sparingly. Shell cordovan does not crack; it rolls. The natural flexibility of the fiber structure means it bends without the surface fracturing the way cowhide can. This is the property that makes it the preferred leather for wallets, dress shoes, and watch straps.