Tanning is the process of making leather, which does not easily decompose, from the skins of animals, which do. Often this uses tannin, an acidic chemical compound. Coloring may occur during tanning. A tannery is the term for a place where these skins are processed.
Tanning leather involves a process which permanently alters the protein structure of skin so that it cannot ever return to rawhide. Making rawhide does not require the use of tannin and is made simply by removing the flesh and fat and then the hair by way of soaking in an aqueous solution (often called liming when using lime and water or bucking when using wood ash (lye) and water), then scraping over a beam with a somewhat dull knife, and then leaving to dry, usually stretched on a frame so that it dries flat. The two aforementioned solutions for removing the hair also act to clean the fiber network of the skin and therefore allow penetration and action of the tanning agent.
The English word for tanning is from medieval Latin tannāre, deriv. of tannum (oak bark), related to Old High German tanna meaning oak or fir (related to modern Tannenbaum). This refers to use of the bark of oaks (the original source of tannin) in some kinds of hide preservation

Tanned leather in Marrakech