When it comes to leather shoes it is helpful to have an understanding of leather in general, and shoe leather in specific.
Most people think of leather as cow hide, but it really relates to any animal skin that has been through the tanning process to convert the dead animal skin into a lasting useful material (Leather).
Although the tanned skin of a young calf is called calfskin it is still leather. Because calfskin comes from a calf it has a tighter grain and fiber, and is thinner and lighter than cow hide; this makes for better shoe leather.
Other types of animal leather are Kidskin (from goat), Pigskin/Peccary (from pig), Cordovan Shell (from horse), and of course other exotic animal skins like buffalo, elephant, kangaroo and so on. There are also bird skins, like ostrich, and reptile skins like alligator, crocodile, lizard and snake.
Reptile skins tend to last longer and need less care than animal leathers, but they are also more expensive. Bovine leather (cow hide / calfskin) is by far the most commonly used leather in shoes.
A high quality all leather shoe uses leather in the following places:
The outsole of the shoe (the part that touches the ground)
The insole of the shoe (the part your foot rests on)
The lining of the shoe (between your foot and the upper)
The heel of the shoe (as in stacked layers of leather to create the heel)
The shoe upper (the rest of the shoe, excluding the items above)
Shoe that are not all leather may have rubber soles, insoles made of various materials, and heels made of wood, rubber or plastic. I would suggest going with all leather if you can, with the exception of perhaps rubber soles if you need to stand in cold wet environments.
A leather outsole on a man’s shoe is around 12oz thickness on average. A leather insole is typically around 14oz in thickness to accommodate the welt. A shoe upper is around 5oz on a typical dress/business shoe, and the lining is about 1oz.
All of these thicknesses can vary due to leather type, welt method, and shoe style. For example Italian shoes tend to be sleeker and therefore use thinner leather in the soles and uppers to achieve the look. Soles that are Blake stitched or bonded don’t require as thick an insole as Goodyear welted shoes.
As a side note: try to avoid bonded or bondwelted shoes as this means the outsole is simply glued to the shoe without an actual welt.
The quality of the leather used in a given line of shoes is determined by the grade of leather the shoe manufacturer purchased to make the shoes. Leather is graded in two basic ways: 1) The quality of the hide in general (amount of scars, blemishes, etc…), and 2) The area of the hide a specific piece of leather is cut from (back, belly, front shoulders, etc…).
Leather quality is typically graded in four grades, with grade 1 being the best, and grade 4 being the worst. This means that even grade 1 hides (little to no blemishes) have grade 4 leather (belly skin).
The grade of leather used is the most critical in the shoe upper, as this is where the quality of the leather is most visible. Shoe uppers made from the back area leather of a grade 1 hide would be the best shoe leather you could get (and also makes for a very expensive pair of shoes).
The leather on a shoe upper is typically grain side out leather, but leathers like shell cordovan and waxed leather are used inside out (flesh side out), and suede leather has had the grain removed entirely.
Inside out leathers are typically pressed under very high pressure to compress the fibers to a smooth surface.
Leather that has blemishes in the grain are often buffed (sanded) of the grain side to remove the blemishes, which then requires the grain to be corrected. Corrected grain leather is sometimes referred to as top grain leather or bookbinder leather. If the grain has not been corrected (no existing blemishes in the grain to begin with) it is referred to as full grain. The term top grain has also been used to define the grain side of the leather, making full grain and top grain synonymous, so it can be confusing.
One of the final stages of tanning leather is applying the color and finish (although chromium tanned leather can be bought in a “wet blue” state).
The high quality leather is typically aniline dyed, which saturates the color completely through the leather. The leather is also pressed under high pressure to give it some shine, and a very thin coat of clear or colored acrylic is applied as a final finish, in most cases.
In the case of corrected grain, the pressing and acrylic finish is also where the corrected grain is applied. Because of this corrected grain leather will have a thicker finish than non-corrected grain, and also tend to be a little shinier. Patent leather is corrected grain leather with a thick acrylic finish, pressed to a high shine.
The shoe manufacturer may also add their own finish to the leather, to add more shine, or to add color highlights.